Merge branch 'yorick/refactor-database-connection-class' into 'master'

Refactor Database::Connection into separate types

See merge request gitlab-org/gitlab!72381

app/models/application_record.rb

 # frozen_string_literal: true
 
 class ApplicationRecord < ActiveRecord::Base
+  include DatabaseReflection
+  include Transactions
+  include LegacyBulkInsert
+
   self.abstract_class = true
 
   alias_method :reset, :reload

app/models/application_setting_implementation.rb

@@ -244,11 +244,11 @@ module ApplicationSettingImplementation
   end
 
   def home_page_url_column_exists?
-    ::Gitlab::Database.main.cached_column_exists?(:application_settings, :home_page_url)
+    ApplicationSetting.database.cached_column_exists?(:home_page_url)
   end
 
   def help_page_support_url_column_exists?
-    ::Gitlab::Database.main.cached_column_exists?(:application_settings, :help_page_support_url)
+    ApplicationSetting.database.cached_column_exists?(:help_page_support_url)
   end
 
   def disabled_oauth_sign_in_sources=(sources)

app/models/concerns/cascading_namespace_setting_attribute.rb

@@ -127,7 +127,7 @@ module CascadingNamespaceSettingAttribute
     end
 
     def alias_boolean(attribute)
-      return unless Gitlab::Database.main.exists? && type_for_attribute(attribute).type == :boolean
+      return unless database.exists? && type_for_attribute(attribute).type == :boolean
 
       alias_method :"#{attribute}?", attribute
     end

app/models/concerns/database_reflection.rb

+# frozen_string_literal: true
+
+# A module that makes it easier/less verbose to reflect upon a database
+# connection.
+#
+# Using this module you can write this:
+#
+#     User.database.database_name
+#
+# Instead of this:
+#
+#     Gitlab::Database::Reflection.new(User).database_name
+module DatabaseReflection
+  extend ActiveSupport::Concern
+
+  class_methods do
+    def database
+      @database_reflection ||= ::Gitlab::Database::Reflection.new(self)
+    end
+  end
+end

app/models/concerns/legacy_bulk_insert.rb

+# frozen_string_literal: true
+
+module LegacyBulkInsert
+  extend ActiveSupport::Concern
+
+  class_methods do
+    # Bulk inserts a number of rows into a table, optionally returning their
+    # IDs.
+    #
+    # This method is deprecated, and you should use the BulkInsertSafe module
+    # instead.
+    #
+    # table - The name of the table to insert the rows into.
+    # rows - An Array of Hash instances, each mapping the columns to their
+    #        values.
+    # return_ids - When set to true the return value will be an Array of IDs of
+    #              the inserted rows
+    # disable_quote - A key or an Array of keys to exclude from quoting (You
+    #                 become responsible for protection from SQL injection for
+    #                 these keys!)
+    # on_conflict - Defines an upsert. Values can be: :disabled (default) or
+    #               :do_nothing
+    def legacy_bulk_insert(table, rows, return_ids: false, disable_quote: [], on_conflict: nil)
+      return if rows.empty?
+
+      keys = rows.first.keys
+      columns = keys.map { |key| connection.quote_column_name(key) }
+
+      disable_quote = Array(disable_quote).to_set
+      tuples = rows.map do |row|
+        keys.map do |k|
+          disable_quote.include?(k) ? row[k] : connection.quote(row[k])
+        end
+      end
+
+      sql = <<-EOF
+        INSERT INTO #{table} (#{columns.join(', ')})
+        VALUES #{tuples.map { |tuple| "(#{tuple.join(', ')})" }.join(', ')}
+      EOF
+
+      sql = "#{sql} ON CONFLICT DO NOTHING" if on_conflict == :do_nothing
+
+      sql = "#{sql} RETURNING id" if return_ids
+
+      result = connection.execute(sql)
+
+      if return_ids
+        result.values.map { |tuple| tuple[0].to_i }
+      else
+        []
+      end
+    end
+  end
+end

app/models/concerns/sha256_attribute.rb

@@ -39,7 +39,7 @@ module Sha256Attribute
     end
 
     def database_exists?
-      Gitlab::Database.main.exists?
+      database.exists?
     end
   end
 end

app/models/concerns/sha_attribute.rb

@@ -32,7 +32,7 @@ module ShaAttribute
     end
 
     def database_exists?
-      Gitlab::Database.main.exists?
+      database.exists?
     end
   end
 end

app/models/concerns/transactions.rb

+# frozen_string_literal: true
+
+module Transactions
+  extend ActiveSupport::Concern
+
+  class_methods do
+    # inside_transaction? will return true if the caller is running within a
+    # transaction. Handles special cases when running inside a test environment,
+    # where tests may be wrapped in transactions
+    def inside_transaction?
+      base = Rails.env.test? ? @open_transactions_baseline.to_i : 0
+
+      connection.open_transactions > base
+    end
+
+    # These methods that access @open_transactions_baseline are not thread-safe.
+    # These are fine though because we only call these in RSpec's main thread.
+    # If we decide to run specs multi-threaded, we would need to use something
+    # like ThreadGroup to keep track of this value
+    def set_open_transactions_baseline
+      @open_transactions_baseline = connection.open_transactions
+    end
+
+    def reset_open_transactions_baseline
+      @open_transactions_baseline = 0
+    end
+  end
+end

app/models/concerns/x509_serial_number_attribute.rb

@@ -39,7 +39,7 @@ module X509SerialNumberAttribute
     end
 
     def database_exists?
-      Gitlab::Database.main.exists?
+      database.exists?
     end
   end
 end

app/models/deployment.rb

@@ -300,7 +300,7 @@ class Deployment < ApplicationRecord
                              "#{id} as deployment_id",
                              "#{environment_id} as environment_id").to_sql
 
-    # We don't use `Gitlab::Database.main.bulk_insert` here so that we don't need to
+    # We don't use `ApplicationRecord.legacy_bulk_insert` here so that we don't need to
     # first pluck lots of IDs into memory.
     #
     # We also ignore any duplicates so this method can be called multiple times

app/models/design_management/version.rb

@@ -88,7 +88,7 @@ module DesignManagement
 
         rows = design_actions.map { |action| action.row_attrs(version) }
 
-        Gitlab::Database.main.bulk_insert(::DesignManagement::Action.table_name, rows) # rubocop:disable Gitlab/BulkInsert
+        ApplicationRecord.legacy_bulk_insert(::DesignManagement::Action.table_name, rows) # rubocop:disable Gitlab/BulkInsert
         version.designs.reset
         version.validate!
         design_actions.each(&:performed)

app/models/merge_request_context_commit.rb

@@ -26,7 +26,7 @@ class MergeRequestContextCommit < ApplicationRecord
 
   # create MergeRequestContextCommit by given commit sha and it's diff file record
   def self.bulk_insert(rows, **args)
-    Gitlab::Database.main.bulk_insert('merge_request_context_commits', rows, **args) # rubocop:disable Gitlab/BulkInsert
+    ApplicationRecord.legacy_bulk_insert('merge_request_context_commits', rows, **args) # rubocop:disable Gitlab/BulkInsert
   end
 
   def to_commit

app/models/merge_request_context_commit_diff_file.rb

@@ -14,7 +14,7 @@ class MergeRequestContextCommitDiffFile < ApplicationRecord
 
   # create MergeRequestContextCommitDiffFile by given diff file record(s)
   def self.bulk_insert(*args)
-    Gitlab::Database.main.bulk_insert('merge_request_context_commit_diff_files', *args) # rubocop:disable Gitlab/BulkInsert
+    ApplicationRecord.legacy_bulk_insert('merge_request_context_commit_diff_files', *args) # rubocop:disable Gitlab/BulkInsert
   end
 
   def path

app/models/merge_request_diff.rb

@@ -515,7 +515,7 @@ class MergeRequestDiff < ApplicationRecord
 
     transaction do
       MergeRequestDiffFile.where(merge_request_diff_id: id).delete_all
-      Gitlab::Database.main.bulk_insert('merge_request_diff_files', rows) # rubocop:disable Gitlab/BulkInsert
+      ApplicationRecord.legacy_bulk_insert('merge_request_diff_files', rows) # rubocop:disable Gitlab/BulkInsert
       save!
     end
 
@@ -535,7 +535,7 @@ class MergeRequestDiff < ApplicationRecord
 
     transaction do
       MergeRequestDiffFile.where(merge_request_diff_id: id).delete_all
-      Gitlab::Database.main.bulk_insert('merge_request_diff_files', rows) # rubocop:disable Gitlab/BulkInsert
+      ApplicationRecord.legacy_bulk_insert('merge_request_diff_files', rows) # rubocop:disable Gitlab/BulkInsert
       update!(stored_externally: false)
     end
 
@@ -595,7 +595,7 @@ class MergeRequestDiff < ApplicationRecord
     rows = build_external_merge_request_diff_files(rows) if use_external_diff?
 
     # Faster inserts
-    Gitlab::Database.main.bulk_insert('merge_request_diff_files', rows) # rubocop:disable Gitlab/BulkInsert
+    ApplicationRecord.legacy_bulk_insert('merge_request_diff_files', rows) # rubocop:disable Gitlab/BulkInsert
   end
 
   def build_external_diff_tempfile(rows)

app/models/merge_request_diff_commit.rb

@@ -74,7 +74,7 @@ class MergeRequestDiffCommit < ApplicationRecord
       )
     end
 
-    Gitlab::Database.main.bulk_insert(self.table_name, rows) # rubocop:disable Gitlab/BulkInsert
+    ApplicationRecord.legacy_bulk_insert(self.table_name, rows) # rubocop:disable Gitlab/BulkInsert
   end
 
   def self.prepare_commits_for_bulk_insert(commits)

app/services/design_management/copy_design_collection/copy_service.rb

@@ -181,12 +181,12 @@ module DesignManagement
             )
           end
 
-          # TODO Replace `Gitlab::Database.main.bulk_insert` with `BulkInsertSafe`
+          # TODO Replace `ApplicationRecord.legacy_bulk_insert` with `BulkInsertSafe`
           # once https://gitlab.com/gitlab-org/gitlab/-/issues/247718 is fixed.
           # When this is fixed, we can remove the call to
           # `with_project_iid_supply` above, since the objects will be instantiated
           # and callbacks (including `ensure_project_iid!`) will fire.
-          ::Gitlab::Database.main.bulk_insert( # rubocop:disable Gitlab/BulkInsert
+          ::ApplicationRecord.legacy_bulk_insert( # rubocop:disable Gitlab/BulkInsert
             DesignManagement::Design.table_name,
             new_rows,
             return_ids: true
@@ -207,9 +207,9 @@ module DesignManagement
           )
         end
 
-        # TODO Replace `Gitlab::Database.main.bulk_insert` with `BulkInsertSafe`
+        # TODO Replace `ApplicationRecord.legacy_bulk_insert` with `BulkInsertSafe`
         # once https://gitlab.com/gitlab-org/gitlab/-/issues/247718 is fixed.
-        ::Gitlab::Database.main.bulk_insert( # rubocop:disable Gitlab/BulkInsert
+        ::ApplicationRecord.legacy_bulk_insert( # rubocop:disable Gitlab/BulkInsert
           DesignManagement::Version.table_name,
           new_rows,
           return_ids: true
@@ -239,7 +239,7 @@ module DesignManagement
         end
 
         # We cannot use `BulkInsertSafe` because of the uploader mounted in `Action`.
-        ::Gitlab::Database.main.bulk_insert( # rubocop:disable Gitlab/BulkInsert
+        ::ApplicationRecord.legacy_bulk_insert( # rubocop:disable Gitlab/BulkInsert
           DesignManagement::Action.table_name,
           new_rows
         )
@@ -278,7 +278,7 @@ module DesignManagement
 
         # We cannot use `BulkInsertSafe` due to the LfsObjectsProject#update_project_statistics
         # callback that fires after_commit.
-        ::Gitlab::Database.main.bulk_insert( # rubocop:disable Gitlab/BulkInsert
+        ::ApplicationRecord.legacy_bulk_insert( # rubocop:disable Gitlab/BulkInsert
           LfsObjectsProject.table_name,
           new_rows,
           on_conflict: :do_nothing # Upsert

app/services/issuable/clone/attributes_rewriter.rb

@@ -99,7 +99,7 @@ module Issuable
             yield(event)
           end.compact
 
-          Gitlab::Database.main.bulk_insert(table_name, events) # rubocop:disable Gitlab/BulkInsert
+          ApplicationRecord.legacy_bulk_insert(table_name, events) # rubocop:disable Gitlab/BulkInsert
         end
       end
 

app/services/packages/create_dependency_service.rb

 # frozen_string_literal: true
 module Packages
+  # rubocop: disable Gitlab/BulkInsert
   class CreateDependencyService < BaseService
     attr_reader :package, :dependencies
 
@@ -51,7 +52,7 @@ module Packages
         }
       end
 
-      ids = database.bulk_insert(Packages::Dependency.table_name, rows, return_ids: true, on_conflict: :do_nothing)
+      ids = ApplicationRecord.legacy_bulk_insert(Packages::Dependency.table_name, rows, return_ids: true, on_conflict: :do_nothing)
       return ids if ids.size == names_and_version_patterns.size
 
       Packages::Dependency.uncached do
@@ -72,11 +73,8 @@ module Packages
         }
       end
 
-      database.bulk_insert(Packages::DependencyLink.table_name, rows)
-    end
-
-    def database
-      ::Gitlab::Database.main
+      ApplicationRecord.legacy_bulk_insert(Packages::DependencyLink.table_name, rows)
     end
   end
+  # rubocop: enable Gitlab/BulkInsert
 end

app/services/packages/nuget/create_dependency_service.rb

@@ -41,7 +41,7 @@ module Packages
           }
         end
 
-        ::Gitlab::Database.main.bulk_insert(::Packages::Nuget::DependencyLinkMetadatum.table_name, rows.compact) # rubocop:disable Gitlab/BulkInsert
+        ::ApplicationRecord.legacy_bulk_insert(::Packages::Nuget::DependencyLinkMetadatum.table_name, rows.compact) # rubocop:disable Gitlab/BulkInsert
       end
 
       def raw_dependency_for(dependency)

app/services/packages/update_tags_service.rb

@@ -15,7 +15,7 @@ module Packages
       tags_to_create = @tags - existing_tags
 
       @package.tags.with_name(tags_to_destroy).delete_all if tags_to_destroy.any?
-      ::Gitlab::Database.main.bulk_insert(Packages::Tag.table_name, rows(tags_to_create)) if tags_to_create.any? # rubocop:disable Gitlab/BulkInsert
+      ::ApplicationRecord.legacy_bulk_insert(Packages::Tag.table_name, rows(tags_to_create)) if tags_to_create.any? # rubocop:disable Gitlab/BulkInsert
     end
 
     private

app/services/projects/detect_repository_languages_service.rb

@@ -21,7 +21,7 @@ module Projects
             .update_all(share: update[:share])
         end
 
-        Gitlab::Database.main.bulk_insert( # rubocop:disable Gitlab/BulkInsert
+        ApplicationRecord.legacy_bulk_insert( # rubocop:disable Gitlab/BulkInsert
           RepositoryLanguage.table_name,
           detection.insertions(matching_programming_languages)
         )

app/services/projects/lfs_pointers/lfs_link_service.rb

@@ -38,7 +38,7 @@ module Projects
           rows = existent_lfs_objects
                    .not_linked_to_project(project)
                    .map { |existing_lfs_object| { project_id: project.id, lfs_object_id: existing_lfs_object.id } }
-          Gitlab::Database.main.bulk_insert(:lfs_objects_projects, rows) # rubocop:disable Gitlab/BulkInsert
+          ApplicationRecord.legacy_bulk_insert(:lfs_objects_projects, rows) # rubocop:disable Gitlab/BulkInsert
           iterations += 1
 
           linked_existing_objects += existent_lfs_objects.map(&:oid)

app/services/resource_events/change_labels_service.rb

@@ -23,7 +23,7 @@ module ResourceEvents
         label_hash.merge(label_id: label.id, action: ResourceLabelEvent.actions['remove'])
       end
 
-      Gitlab::Database.main.bulk_insert(ResourceLabelEvent.table_name, labels) # rubocop:disable Gitlab/BulkInsert
+      ApplicationRecord.legacy_bulk_insert(ResourceLabelEvent.table_name, labels) # rubocop:disable Gitlab/BulkInsert
       resource.expire_note_etag_cache
 
       Gitlab::UsageDataCounters::IssueActivityUniqueCounter.track_issue_label_changed_action(author: user) if resource.is_a?(Issue)

app/services/suggestions/create_service.rb

@@ -25,7 +25,7 @@ module Suggestions
         end
 
       rows.in_groups_of(100, false) do |rows|
-        Gitlab::Database.main.bulk_insert('suggestions', rows) # rubocop:disable Gitlab/BulkInsert
+        ApplicationRecord.legacy_bulk_insert('suggestions', rows) # rubocop:disable Gitlab/BulkInsert
       end
 
       Gitlab::UsageDataCounters::MergeRequestActivityUniqueCounter.track_add_suggestion_action(note: @note)

app/views/admin/dashboard/index.html.haml

@@ -153,9 +153,9 @@
             %span.float-right
               #{Rails::VERSION::STRING}
           %p
-            = Gitlab::Database.main.human_adapter_name
+            = ApplicationRecord.database.human_adapter_name
             %span.float-right
-              = Gitlab::Database.main.version
+              = ApplicationRecord.database.version
           %p
             = _('Redis')
             %span.float-right

app/workers/gitlab/jira_import/import_issue_worker.rb

@@ -54,7 +54,7 @@ module Gitlab
 
         label_link_attrs << build_label_attrs(issue_id, import_label_id.to_i)
 
-        Gitlab::Database.main.bulk_insert(LabelLink.table_name, label_link_attrs) # rubocop:disable Gitlab/BulkInsert
+        ApplicationRecord.legacy_bulk_insert(LabelLink.table_name, label_link_attrs) # rubocop:disable Gitlab/BulkInsert
       end
 
       def assign_issue(project_id, issue_id, assignee_ids)
@@ -62,7 +62,7 @@ module Gitlab
 
         assignee_attrs = assignee_ids.map { |user_id| { issue_id: issue_id, user_id: user_id } }
 
-        Gitlab::Database.main.bulk_insert(IssueAssignee.table_name, assignee_attrs) # rubocop:disable Gitlab/BulkInsert
+        ApplicationRecord.legacy_bulk_insert(IssueAssignee.table_name, assignee_attrs) # rubocop:disable Gitlab/BulkInsert
       end
 
       def build_label_attrs(issue_id, label_id)

config/initializers/1_postgresql_only.rb

 # frozen_string_literal: true
 
 raise "PostgreSQL is the only supported database from GitLab 12.1" unless
-  Gitlab::Database.main.postgresql?
+  ApplicationRecord.database.postgresql?
 
 Gitlab::Database.check_postgres_version_and_print_warning

config/initializers/active_record_lifecycle.rb

@@ -14,7 +14,7 @@ end
 
 if defined?(ActiveRecord::Base)
   Gitlab::Cluster::LifecycleEvents.on_before_fork do
-    raise 'ActiveRecord connection not established. Unable to start.' unless Gitlab::Database.main.exists?
+    raise 'ActiveRecord connection not established. Unable to start.' unless ApplicationRecord.database.exists?
 
     # the following is highly recommended for Rails + "preload_app true"
     # as there's no need for the master process to hold a connection

config/initializers/console_message.rb

@@ -10,8 +10,8 @@ if Gitlab::Runtime.console?
   puts " GitLab:".ljust(justify) + "#{Gitlab::VERSION} (#{Gitlab.revision}) #{Gitlab.ee? ? 'EE' : 'FOSS'}"
   puts " GitLab Shell:".ljust(justify) + "#{Gitlab::VersionInfo.parse(Gitlab::Shell.version)}"
 
-  if Gitlab::Database.main.exists?
-    puts " #{Gitlab::Database.main.human_adapter_name}:".ljust(justify) + Gitlab::Database.main.version
+  if ApplicationRecord.database.exists?
+    puts " #{ApplicationRecord.database.human_adapter_name}:".ljust(justify) + ApplicationRecord.database.version
 
     Gitlab.ee do
       if Gitlab::Geo.connected? && Gitlab::Geo.enabled?

config/initializers/forbid_sidekiq_in_transactions.rb

@@ -20,7 +20,7 @@ module Sidekiq
       module NoEnqueueingFromTransactions
         %i(perform_async perform_at perform_in).each do |name|
           define_method(name) do |*args|
-            if !Sidekiq::Worker.skip_transaction_check && Gitlab::Database.main.inside_transaction?
+            if !Sidekiq::Worker.skip_transaction_check && ApplicationRecord.inside_transaction?
               begin
                 raise Sidekiq::Worker::EnqueueFromTransactionError, <<~MSG
                 `#{self}.#{name}` cannot be called inside a transaction as this can lead to

db/post_migrate/20200310215714_migrate_saml_identities_to_scim_identities.rb

@@ -20,7 +20,7 @@ class MigrateSamlIdentitiesToScimIdentities < ActiveRecord::Migration[6.0]
           record.attributes.extract!("extern_uid", "user_id", "group_id", "active", "created_at", "updated_at")
         end
 
-        Gitlab::Database.main.bulk_insert(:scim_identities, data_to_insert, on_conflict: :do_nothing) # rubocop:disable Gitlab/BulkInsert
+        ApplicationRecord.legacy_bulk_insert(:scim_identities, data_to_insert, on_conflict: :do_nothing) # rubocop:disable Gitlab/BulkInsert
       end
   end
 

doc/development/verifying_database_capabilities.md

@@ -12,13 +12,13 @@ necessary to add database (version) specific behavior.
 
 To facilitate this we have the following methods that you can use:
 
-- `Gitlab::Database.main.version`: returns the PostgreSQL version number as a string
+- `ApplicationRecord.database.version`: returns the PostgreSQL version number as a string
   in the format `X.Y.Z`.
 
 This allows you to write code such as:
 
 ```ruby
-if Gitlab::Database.main.version.to_f >= 11.7
+if ApplicationRecord.database.version.to_f >= 11.7
   run_really_fast_query
 else
   run_fast_query

ee/app/models/ee/application_setting.rb

@@ -421,19 +421,19 @@ module EE
     end
 
     def elasticsearch_indexing_column_exists?
-      ::Gitlab::Database.main.cached_column_exists?(:application_settings, :elasticsearch_indexing)
+      self.class.database.cached_column_exists?(:elasticsearch_indexing)
     end
 
     def elasticsearch_pause_indexing_column_exists?
-      ::Gitlab::Database.main.cached_column_exists?(:application_settings, :elasticsearch_pause_indexing)
+      self.class.database.cached_column_exists?(:elasticsearch_pause_indexing)
     end
 
     def elasticsearch_search_column_exists?
-      ::Gitlab::Database.main.cached_column_exists?(:application_settings, :elasticsearch_search)
+      self.class.database.cached_column_exists?(:elasticsearch_search)
     end
 
     def email_additional_text_column_exists?
-      ::Gitlab::Database.main.cached_column_exists?(:application_settings, :email_additional_text)
+      self.class.database.cached_column_exists?(:email_additional_text)
     end
 
     def check_geo_node_allowed_ips

ee/app/models/elasticsearch_indexed_namespace.rb

@@ -37,7 +37,7 @@ class ElasticsearchIndexedNamespace < ApplicationRecord
         { created_at: now, updated_at: now, namespace_id: id }
       end
 
-      Gitlab::Database.main.bulk_insert(table_name, insert_rows) # rubocop:disable Gitlab/BulkInsert
+      ApplicationRecord.legacy_bulk_insert(table_name, insert_rows) # rubocop:disable Gitlab/BulkInsert
       invalidate_elasticsearch_indexes_cache!
 
       jobs = batch_ids.map { |id| [id, :index] }

ee/app/services/audit_events/bulk_insert_service.rb

@@ -18,7 +18,7 @@ module AuditEvents
       return if collection.empty?
 
       collection.in_groups_of(BATCH_SIZE, false) do |services|
-        ::Gitlab::Database.main.bulk_insert(::AuditEvent.table_name, services.map(&:attributes)) # rubocop:disable Gitlab/BulkInsert
+        ::ApplicationRecord.legacy_bulk_insert(::AuditEvent.table_name, services.map(&:attributes)) # rubocop:disable Gitlab/BulkInsert
 
         services.each(&:log_security_event_to_file)
       end

ee/app/services/iterations/cadences/create_iterations_in_advance_service.rb

@@ -15,7 +15,7 @@ module Iterations
         return ::ServiceResponse.error(message: _('Cadence is not automated'), http_status: 422) unless cadence.can_be_automated?
 
         update_existing_iterations!
-        ::Gitlab::Database.main.bulk_insert(Iteration.table_name, build_new_iterations) # rubocop:disable Gitlab/BulkInsert
+        ::ApplicationRecord.legacy_bulk_insert(Iteration.table_name, build_new_iterations) # rubocop:disable Gitlab/BulkInsert
 
         cadence.update!(last_run_date: compute_last_run_date)
 

ee/app/services/iterations/roll_over_issues_service.rb

@@ -20,8 +20,8 @@ module Iterations
 
         ApplicationRecord.transaction do
           issues.update_all(sprint_id: to_iteration.id, updated_at: rolled_over_at)
-          Gitlab::Database.main.bulk_insert(ResourceIterationEvent.table_name, remove_iteration_events) # rubocop:disable Gitlab/BulkInsert
-          Gitlab::Database.main.bulk_insert(ResourceIterationEvent.table_name, add_iteration_events) # rubocop:disable Gitlab/BulkInsert
+          ApplicationRecord.legacy_bulk_insert(ResourceIterationEvent.table_name, remove_iteration_events) # rubocop:disable Gitlab/BulkInsert
+          ApplicationRecord.legacy_bulk_insert(ResourceIterationEvent.table_name, add_iteration_events) # rubocop:disable Gitlab/BulkInsert
         end
       end
 

ee/app/services/resource_events/change_weight_service.rb

@@ -10,7 +10,7 @@ module ResourceEvents
     end
 
     def execute
-      ::Gitlab::Database.main.bulk_insert(ResourceWeightEvent.table_name, resource_weight_changes) # rubocop:disable Gitlab/BulkInsert
+      ::ApplicationRecord.legacy_bulk_insert(ResourceWeightEvent.table_name, resource_weight_changes) # rubocop:disable Gitlab/BulkInsert
       resource.expire_note_etag_cache
 
       Gitlab::UsageDataCounters::IssueActivityUniqueCounter.track_issue_weight_changed_action(author: user)

ee/app/workers/geo/scheduler/scheduler_worker.rb

@@ -163,7 +163,7 @@ module Geo
 
       def update_pending_resources
         if reload_queue?
-          @pending_resources = Gitlab::Database.main.geo_uncached_queries { load_pending_resources }
+          @pending_resources = Gitlab::Geo.uncached_queries { load_pending_resources }
           set_backoff_time! if should_apply_backoff?
         end
       end

ee/lib/ee/gitlab/background_migration/generate_gitlab_subscriptions.rb

@@ -49,7 +49,7 @@ module EE
                     }
                   end
 
-          Gitlab::Database.main.bulk_insert(:gitlab_subscriptions, rows) # rubocop:disable Gitlab/BulkInsert
+          ApplicationRecord.legacy_bulk_insert(:gitlab_subscriptions, rows) # rubocop:disable Gitlab/BulkInsert
         end
       end
     end

ee/lib/ee/gitlab/database/connection.rb

-# frozen_string_literal: true
-
-module EE
-  module Gitlab
-    module Database
-      module Connection
-        extend ActiveSupport::Concern
-
-        def geo_uncached_queries(&block)
-          raise 'No block given' unless block_given?
-
-          scope.uncached do
-            if ::Gitlab::Geo.secondary?
-              Geo::TrackingBase.uncached(&block)
-            else
-              yield
-            end
-          end
-        end
-      end
-    end
-  end
-end

ee/lib/ee/gitlab/import_sources.rb

@@ -14,7 +14,7 @@ module EE
         # This method can be called/loaded before the database
         # has been created. With this guard clause we prevent querying
         # the License table until the table exists
-        return [] unless ::Gitlab::Database.main.cached_table_exists?('licenses') &&
+        return [] unless License.database.cached_table_exists? &&
           License.feature_available?(:custom_project_templates)
 
         [::Gitlab::ImportSources::ImportSource.new('gitlab_custom_project_template',

ee/lib/ee/gitlab/middleware/read_only/controller.rb

@@ -83,7 +83,7 @@ module EE
             action = route_hash[:action]
 
             if ALLOWLISTED_GEO_ROUTES[controller]&.include?(action)
-              ::Gitlab::Database.main.db_read_write?
+              ::ApplicationRecord.database.db_read_write?
             else
               ALLOWLISTED_GEO_ROUTES_TRACKING_DB[controller]&.include?(action)
             end

ee/lib/gitlab/geo.rb

@@ -213,5 +213,17 @@ module Gitlab
 
       [1, capacity].max # at least 1
     end
+
+    def self.uncached_queries(&block)
+      raise 'No block given' unless block_given?
+
+      ApplicationRecord.uncached do
+        if ::Gitlab::Geo.secondary?
+          ::Geo::TrackingBase.uncached(&block)
+        else
+          yield
+        end
+      end
+    end
   end
 end

ee/lib/gitlab/geo/health_check.rb

@@ -9,7 +9,7 @@ module Gitlab
         return '' unless Gitlab::Geo.secondary?
         return 'Geo database configuration file is missing.' unless Gitlab::Geo.geo_database_configured?
         return 'An existing tracking database cannot be reused.' if reusing_existing_tracking_database?
-        return 'Geo node has a database that is writable which is an indication it is not configured for replication with the primary node.' unless Gitlab::Database.main.db_read_only?
+        return 'Geo node has a database that is writable which is an indication it is not configured for replication with the primary node.' unless ApplicationRecord.database.db_read_only?
         return 'Geo node does not appear to be replicating the database from the primary node.' if replication_enabled? && !replication_working?
         return "Geo database version (#{database_version}) does not match latest migration (#{migration_version}).\nYou may have to run `gitlab-rake geo:db:migrate` as root on the secondary." unless database_migration_version_match?
 

ee/spec/lib/ee/gitlab/database/connection_spec.rb

-# frozen_string_literal: true
-
-require 'spec_helper'
-
-RSpec.describe Gitlab::Database::Connection do
-  include ::EE::GeoHelpers
-
-  let(:connection) { described_class.new }
-
-  describe '#geo_uncached_queries' do
-    context 'when no block is given' do
-      it 'raises error' do
-        expect do
-          connection.geo_uncached_queries
-        end.to raise_error('No block given')
-      end
-    end
-
-    context 'when the current node is a primary' do
-      let!(:primary) { create(:geo_node, :primary) }
-
-      it 'wraps the block in an ActiveRecord::Base.uncached block' do
-        stub_current_geo_node(primary)
-
-        expect(Geo::TrackingBase).not_to receive(:uncached)
-        expect(ActiveRecord::Base).to receive(:uncached).and_call_original
-
-        expect do |b|
-          connection.geo_uncached_queries(&b)
-        end.to yield_control
-      end
-    end
-
-    context 'when the current node is a secondary' do
-      let!(:primary) { create(:geo_node, :primary) }
-      let!(:secondary) { create(:geo_node) }
-
-      it 'wraps the block in a Geo::TrackingBase.uncached block and an ActiveRecord::Base.uncached block' do
-        stub_current_geo_node(secondary)
-
-        expect(Geo::TrackingBase).to receive(:uncached).and_call_original
-        expect(ActiveRecord::Base).to receive(:uncached).and_call_original
-
-        expect do |b|
-          connection.geo_uncached_queries(&b)
-        end.to yield_control
-      end
-    end
-
-    context 'when there is no current node' do
-      it 'wraps the block in an ActiveRecord::Base.uncached block' do
-        expect(Geo::TrackingBase).not_to receive(:uncached)
-        expect(ActiveRecord::Base).to receive(:uncached).and_call_original
-
-        expect do |b|
-          connection.geo_uncached_queries(&b)
-        end.to yield_control
-      end
-    end
-  end
-end

ee/spec/lib/gitlab/geo/health_check_spec.rb

@@ -37,7 +37,7 @@ RSpec.describe Gitlab::Geo::HealthCheck, :geo do
         before do
           allow(Gitlab::Geo).to receive(:secondary?) { true }
           allow(Gitlab::Geo).to receive(:geo_database_configured?) { geo_database_configured }
-          allow(Gitlab::Database.main).to receive(:db_read_only?) { db_read_only }
+          allow(ApplicationRecord.database).to receive(:db_read_only?) { db_read_only }
         end
 
         context 'when the Geo tracking DB is not configured' do
@@ -124,8 +124,8 @@ RSpec.describe Gitlab::Geo::HealthCheck, :geo do
   describe '#db_replication_lag_seconds' do
     before do
       query = 'SELECT CASE WHEN pg_last_wal_receive_lsn() = pg_last_wal_replay_lsn() THEN 0 ELSE EXTRACT (EPOCH FROM now() - pg_last_xact_replay_timestamp())::INTEGER END AS replication_lag'
-      allow(Gitlab::Database.main).to receive(:pg_last_wal_receive_lsn).and_return('pg_last_wal_receive_lsn')
-      allow(Gitlab::Database.main).to receive(:pg_last_wal_replay_lsn).and_return('pg_last_wal_replay_lsn')
+      allow(ApplicationRecord.database).to receive(:pg_last_wal_receive_lsn).and_return('pg_last_wal_receive_lsn')
+      allow(ApplicationRecord.database).to receive(:pg_last_wal_replay_lsn).and_return('pg_last_wal_replay_lsn')
       allow(ActiveRecord::Base).to receive_message_chain('connection.execute').with(query).and_return([{ 'replication_lag' => lag_in_seconds }])
     end
 

ee/spec/lib/gitlab/geo_spec.rb

@@ -426,4 +426,51 @@ RSpec.describe Gitlab::Geo, :geo, :request_store do
       end
     end
   end
+
+  describe '.uncached_queries' do
+    context 'when no block is given' do
+      it 'raises error' do
+        expect do
+          described_class.uncached_queries
+        end.to raise_error('No block given')
+      end
+    end
+
+    context 'when the current node is a primary' do
+      it 'wraps the block in an ApplicationRecord.uncached block' do
+        stub_current_geo_node(primary_node)
+
+        expect(Geo::TrackingBase).not_to receive(:uncached)
+        expect(ApplicationRecord).to receive(:uncached).and_call_original
+
+        expect do |b|
+          described_class.uncached_queries(&b)
+        end.to yield_control
+      end
+    end
+
+    context 'when the current node is a secondary' do
+      it 'wraps the block in a Geo::TrackingBase.uncached block and an ApplicationRecord.uncached block' do
+        stub_current_geo_node(secondary_node)
+
+        expect(Geo::TrackingBase).to receive(:uncached).and_call_original
+        expect(ApplicationRecord).to receive(:uncached).and_call_original
+
+        expect do |b|
+          described_class.uncached_queries(&b)
+        end.to yield_control
+      end
+    end
+
+    context 'when there is no current node' do
+      it 'wraps the block in an ApplicationRecord.uncached block' do
+        expect(Geo::TrackingBase).not_to receive(:uncached)
+        expect(ApplicationRecord).to receive(:uncached).and_call_original
+
+        expect do |b|
+          described_class.uncached_queries(&b)
+        end.to yield_control
+      end
+    end
+  end
 end

lib/after_commit_queue.rb

@@ -15,7 +15,7 @@ module AfterCommitQueue
   end
 
   def run_after_commit_or_now(&block)
-    if Gitlab::Database.main.inside_transaction?
+    if ApplicationRecord.inside_transaction?
       if ActiveRecord::Base.connection.current_transaction.records&.include?(self)
         run_after_commit(&block)
       else

lib/feature.rb

@@ -6,6 +6,8 @@ require 'flipper/adapters/active_support_cache_store'
 class Feature
   # Classes to override flipper table names
   class FlipperFeature < Flipper::Adapters::ActiveRecord::Feature
+    include DatabaseReflection
+
     # Using `self.table_name` won't work. ActiveRecord bug?
     superclass.table_name = 'features'
 
@@ -36,7 +38,7 @@ class Feature
     end
 
     def persisted_names
-      return [] unless Gitlab::Database.main.exists?
+      return [] unless ApplicationRecord.database.exists?
 
       # This loads names of all stored feature flags
       # and returns a stable Set in the following order:
@@ -73,7 +75,7 @@ class Feature
 
       # During setup the database does not exist yet. So we haven't stored a value
       # for the feature yet and return the default.
-      return default_enabled unless Gitlab::Database.main.exists?
+      return default_enabled unless ApplicationRecord.database.exists?
 
       feature = get(key)
 

lib/feature/gitaly.rb

@@ -15,7 +15,7 @@ class Feature
 
       def server_feature_flags(project = nil)
         # We need to check that both the DB connection and table exists
-        return {} unless ::Gitlab::Database.main.cached_table_exists?(FlipperFeature.table_name)
+        return {} unless FlipperFeature.database.cached_table_exists?
 
         Feature.persisted_names
           .select { |f| f.start_with?(PREFIX) }

lib/gitlab/background_migration/backfill_project_repositories.rb

@@ -189,7 +189,7 @@ module Gitlab
       end
 
       def perform(start_id, stop_id)
-        Gitlab::Database.main.bulk_insert(:project_repositories, project_repositories(start_id, stop_id)) # rubocop:disable Gitlab/BulkInsert
+        ApplicationRecord.legacy_bulk_insert(:project_repositories, project_repositories(start_id, stop_id)) # rubocop:disable Gitlab/BulkInsert
       end
 
       private

lib/gitlab/background_migration/job_coordinator.rb

@@ -120,7 +120,10 @@ module Gitlab
       end
 
       def connection
-        @connection ||= Gitlab::Database.databases.fetch(database, Gitlab::Database.main).scope.connection
+        @connection ||= Gitlab::Database
+          .database_base_models
+          .fetch(database, Gitlab::Database::PRIMARY_DATABASE_NAME)
+          .connection
       end
     end
   end

lib/gitlab/background_migration/migrate_fingerprint_sha256_within_keys.rb

@@ -34,7 +34,7 @@ module Gitlab
             end
           end
 
-          Gitlab::Database.main.bulk_insert(TEMP_TABLE, fingerprints) # rubocop:disable Gitlab/BulkInsert
+          ApplicationRecord.legacy_bulk_insert(TEMP_TABLE, fingerprints) # rubocop:disable Gitlab/BulkInsert
 
           execute("ANALYZE #{TEMP_TABLE}")
 

lib/gitlab/background_migration/migrate_issue_trackers_sensitive_data.rb

@@ -65,7 +65,7 @@ module Gitlab
           next if service_ids.empty?
 
           migrated_ids += service_ids
-          Gitlab::Database.main.bulk_insert(table, data) # rubocop:disable Gitlab/BulkInsert
+          ApplicationRecord.legacy_bulk_insert(table, data) # rubocop:disable Gitlab/BulkInsert
         end
 
         return if migrated_ids.empty?

lib/gitlab/background_migration/populate_issue_email_participants.rb

@@ -21,7 +21,7 @@ module Gitlab
           }
         end
 
-        Gitlab::Database.main.bulk_insert(:issue_email_participants, rows, on_conflict: :do_nothing) # rubocop:disable Gitlab/BulkInsert
+        ApplicationRecord.legacy_bulk_insert(:issue_email_participants, rows, on_conflict: :do_nothing) # rubocop:disable Gitlab/BulkInsert
       end
     end
   end

lib/gitlab/config_checker/external_database_checker.rb

@@ -6,7 +6,7 @@ module Gitlab
       extend self
 
       def check
-        return [] if Gitlab::Database.main.postgresql_minimum_supported_version?
+        return [] if ApplicationRecord.database.postgresql_minimum_supported_version?
 
         [
           {
@@ -15,7 +15,7 @@ module Gitlab
                        '%{pg_version_minimum} is required for this version of GitLab. ' \
                        'Please upgrade your environment to a supported PostgreSQL version, ' \
                        'see %{pg_requirements_url} for details.') % {
-                                                                      pg_version_current: Gitlab::Database.main.version,
+                                                                      pg_version_current: ApplicationRecord.database.version,
                                                                       pg_version_minimum: Gitlab::Database::MINIMUM_POSTGRES_VERSION,
                                                                       pg_requirements_url: '<a href="https://docs.gitlab.com/ee/install/requirements.html#database">database requirements</a>'
                                                                     }

lib/gitlab/current_settings.rb

@@ -85,7 +85,7 @@ module Gitlab
         active_db_connection = ActiveRecord::Base.connection.active? rescue false
 
         active_db_connection &&
-          Gitlab::Database.main.cached_table_exists?('application_settings')
+          ApplicationSetting.database.cached_table_exists?
       rescue ActiveRecord::NoDatabaseError
         false
       end

lib/gitlab/database.rb

@@ -60,18 +60,7 @@ module Gitlab
         # inherit from ApplicationRecord.
         main: ::ActiveRecord::Base,
         ci: ::Ci::ApplicationRecord.connection_class? ? ::Ci::ApplicationRecord : nil
-      }.compact.freeze
-    end
-
-    def self.databases
-      @databases ||= database_base_models
-        .transform_values { |connection_class| Connection.new(connection_class) }
-        .with_indifferent_access
-        .freeze
-    end
-
-    def self.main
-      databases[PRIMARY_DATABASE_NAME]
+      }.compact.with_indifferent_access.freeze
     end
 
     # We configure the database connection pool size automatically based on the
@@ -110,8 +99,10 @@ module Gitlab
     def self.check_postgres_version_and_print_warning
       return if Gitlab::Runtime.rails_runner?
 
-      databases.each do |name, connection|
-        next if connection.postgresql_minimum_supported_version?
+      database_base_models.each do |name, model|
+        database = Gitlab::Database::Reflection.new(model)
+
+        next if database.postgresql_minimum_supported_version?
 
         Kernel.warn ERB.new(Rainbow.new.wrap(<<~EOS).red).result
 
@@ -122,7 +113,7 @@ module Gitlab
                      ███ ███  ██   ██ ██   ██ ██   ████ ██ ██   ████  ██████  
 
           ******************************************************************************
-            You are using PostgreSQL #{connection.version} for the #{name} database, but PostgreSQL >= <%= Gitlab::Database::MINIMUM_POSTGRES_VERSION %>
+            You are using PostgreSQL #{database.version} for the #{name} database, but PostgreSQL >= <%= Gitlab::Database::MINIMUM_POSTGRES_VERSION %>
             is required for this version of GitLab.
             <% if Rails.env.development? || Rails.env.test? %>
             If using gitlab-development-kit, please find the relevant steps here:

lib/gitlab/database/as_with_materialized.rb

@@ -19,7 +19,7 @@ module Gitlab
       # Note: to be deleted after the minimum PG version is set to 12.0
       def self.materialized_supported?
         strong_memoize(:materialized_supported) do
-          Gitlab::Database.main.version.match?(/^1[2-9]\./) # version 12.x and above
+          ApplicationRecord.database.version.match?(/^1[2-9]\./) # version 12.x and above
         end
       end
 

lib/gitlab/database/each_database.rb

@@ -5,8 +5,8 @@ module Gitlab
     module EachDatabase
       class << self
         def each_database_connection
-          Gitlab::Database.databases.each_pair do |connection_name, connection_wrapper|
-            connection = connection_wrapper.scope.connection
+          Gitlab::Database.database_base_models.each_pair do |connection_name, model|
+            connection = model.connection
 
             with_shared_connection(connection, connection_name) do
               yield connection, connection_name

lib/gitlab/database/load_balancing/load_balancer.rb

@@ -263,6 +263,21 @@ module Gitlab
           ) || raise(::ActiveRecord::ConnectionNotEstablished)
         end
 
+        def wal_diff(location1, location2)
+          read_write do |connection|
+            lsn1 = connection.quote(location1)
+            lsn2 = connection.quote(location2)
+
+            query = <<-SQL.squish
+            SELECT pg_wal_lsn_diff(#{lsn1}, #{lsn2})
+              AS result
+            SQL
+
+            row = connection.select_all(query).first
+            row['result'] if row
+          end
+        end
+
         private
 
         def ensure_caching!

lib/gitlab/database/migration_helpers.rb

@@ -1260,8 +1260,8 @@ module Gitlab
 
       def check_trigger_permissions!(table)
         unless Grant.create_and_execute_trigger?(table)
-          dbname = Database.main.database_name
-          user = Database.main.username
+          dbname = ApplicationRecord.database.database_name
+          user = ApplicationRecord.database.username
 
           raise <<-EOF
 Your database user is not allowed to create, drop, or execute triggers on the
@@ -1583,8 +1583,8 @@ into similar problems in the future (e.g. when new tables are created).
       def create_extension(extension)
         execute('CREATE EXTENSION IF NOT EXISTS %s' % extension)
       rescue ActiveRecord::StatementInvalid => e
-        dbname = Database.main.database_name
-        user = Database.main.username
+        dbname = ApplicationRecord.database.database_name
+        user = ApplicationRecord.database.username
 
         warn(<<~MSG) if e.to_s =~ /permission denied/
           GitLab requires the PostgreSQL extension '#{extension}' installed in database '#{dbname}', but
@@ -1611,8 +1611,8 @@ into similar problems in the future (e.g. when new tables are created).
       def drop_extension(extension)
         execute('DROP EXTENSION IF EXISTS %s' % extension)
       rescue ActiveRecord::StatementInvalid => e
-        dbname = Database.main.database_name
-        user = Database.main.username
+        dbname = ApplicationRecord.database.database_name
+        user = ApplicationRecord.database.username
 
         warn(<<~MSG) if e.to_s =~ /permission denied/
           This migration attempts to drop the PostgreSQL extension '#{extension}'

lib/gitlab/database/connection.rb → lib/gitlab/database/reflection.rb

@@ -2,20 +2,14 @@
 
 module Gitlab
   module Database
-    # Configuration settings and methods for interacting with a PostgreSQL
-    # database, with support for multiple databases.
-    class Connection
-      attr_reader :scope
-
-      # Initializes a new `Database`.
-      #
-      # The `scope` argument must be an object (such as `ActiveRecord::Base`)
-      # that supports retrieving connections and connection pools.
-      def initialize(scope = ActiveRecord::Base)
-        @config = nil
-        @scope = scope
+    # A class for reflecting upon a database and its settings, such as the
+    # adapter name, PostgreSQL version, and the presence of tables or columns.
+    class Reflection
+      attr_reader :model
+
+      def initialize(model)
+        @model = model
         @version = nil
-        @open_transactions_baseline = 0
       end
 
       def config
@@ -28,11 +22,7 @@ module Gitlab
         #
         # - https://gitlab.com/gitlab-org/release/retrospectives/-/issues/39
         # - https://gitlab.com/gitlab-com/gl-infra/production/-/issues/5238
-        scope.connection_db_config.configuration_hash.with_indifferent_access
-      end
-
-      def pool_size
-        config[:pool] || Database.default_pool_size
+        model.connection_db_config.configuration_hash.with_indifferent_access
       end
 
       def username
@@ -62,8 +52,7 @@ module Gitlab
       # Check whether the underlying database is in read-only mode
       def db_read_only?
         pg_is_in_recovery =
-          scope
-            .connection
+          connection
             .execute('SELECT pg_is_in_recovery()')
             .first
             .fetch('pg_is_in_recovery')
@@ -87,58 +76,14 @@ module Gitlab
         version.to_f >= MINIMUM_POSTGRES_VERSION
       end
 
-      # Bulk inserts a number of rows into a table, optionally returning their
-      # IDs.
-      #
-      # table - The name of the table to insert the rows into.
-      # rows - An Array of Hash instances, each mapping the columns to their
-      #        values.
-      # return_ids - When set to true the return value will be an Array of IDs of
-      #              the inserted rows
-      # disable_quote - A key or an Array of keys to exclude from quoting (You
-      #                 become responsible for protection from SQL injection for
-      #                 these keys!)
-      # on_conflict - Defines an upsert. Values can be: :disabled (default) or
-      #               :do_nothing
-      def bulk_insert(table, rows, return_ids: false, disable_quote: [], on_conflict: nil)
-        return if rows.empty?
-
-        keys = rows.first.keys
-        columns = keys.map { |key| connection.quote_column_name(key) }
-
-        disable_quote = Array(disable_quote).to_set
-        tuples = rows.map do |row|
-          keys.map do |k|
-            disable_quote.include?(k) ? row[k] : connection.quote(row[k])
-          end
-        end
-
-        sql = <<-EOF
-          INSERT INTO #{table} (#{columns.join(', ')})
-          VALUES #{tuples.map { |tuple| "(#{tuple.join(', ')})" }.join(', ')}
-        EOF
-
-        sql = "#{sql} ON CONFLICT DO NOTHING" if on_conflict == :do_nothing
-
-        sql = "#{sql} RETURNING id" if return_ids
-
-        result = connection.execute(sql)
-
-        if return_ids
-          result.values.map { |tuple| tuple[0].to_i }
-        else
-          []
-        end
-      end
-
-      def cached_column_exists?(table_name, column_name)
+      def cached_column_exists?(column_name)
         connection
-          .schema_cache.columns_hash(table_name)
+          .schema_cache.columns_hash(model.table_name)
           .has_key?(column_name.to_s)
       end
 
-      def cached_table_exists?(table_name)
-        exists? && connection.schema_cache.data_source_exists?(table_name)
+      def cached_table_exists?
+        exists? && connection.schema_cache.data_source_exists?(model.table_name)
       end
 
       def exists?
@@ -160,47 +105,11 @@ module Gitlab
         row['system_identifier']
       end
 
-      def pg_wal_lsn_diff(location1, location2)
-        lsn1 = connection.quote(location1)
-        lsn2 = connection.quote(location2)
-
-        query = <<-SQL.squish
-            SELECT pg_wal_lsn_diff(#{lsn1}, #{lsn2})
-              AS result
-        SQL
-
-        row = connection.select_all(query).first
-        row['result'] if row
-      end
-
-      # inside_transaction? will return true if the caller is running within a
-      # transaction. Handles special cases when running inside a test
-      # environment, where tests may be wrapped in transactions
-      def inside_transaction?
-        base = Rails.env.test? ? @open_transactions_baseline : 0
-
-        scope.connection.open_transactions > base
-      end
-
-      # These methods that access @open_transactions_baseline are not
-      # thread-safe.  These are fine though because we only call these in
-      # RSpec's main thread. If we decide to run specs multi-threaded, we would
-      # need to use something like ThreadGroup to keep track of this value
-      def set_open_transactions_baseline
-        @open_transactions_baseline = scope.connection.open_transactions
-      end
-
-      def reset_open_transactions_baseline
-        @open_transactions_baseline = 0
-      end
-
       private
 
       def connection
-        scope.connection
+        model.connection
       end
     end
   end
 end
-
-Gitlab::Database::Connection.prepend_mod_with('Gitlab::Database::Connection')

lib/gitlab/github_import/bulk_importing.rb

@@ -30,7 +30,7 @@ module Gitlab
       # Bulk inserts the given rows into the database.
       def bulk_insert(model, rows, batch_size: 100)
         rows.each_slice(batch_size) do |slice|
-          Gitlab::Database.main.bulk_insert(model.table_name, slice) # rubocop:disable Gitlab/BulkInsert
+          ApplicationRecord.legacy_bulk_insert(model.table_name, slice) # rubocop:disable Gitlab/BulkInsert
 
           log_and_increment_counter(slice.size, :imported)
         end

lib/gitlab/github_import/importer/diff_note_importer.rb

@@ -70,7 +70,7 @@ module Gitlab
           # To work around this we're using bulk_insert with a single row. This
           # allows us to efficiently insert data (even if it's just 1 row)
           # without having to use all sorts of hacks to disable callbacks.
-          Gitlab::Database.main.bulk_insert(LegacyDiffNote.table_name, [{
+          ApplicationRecord.legacy_bulk_insert(LegacyDiffNote.table_name, [{
             noteable_type: note.noteable_type,
             system: false,
             type: 'LegacyDiffNote',

lib/gitlab/github_import/importer/issue_importer.rb

@@ -75,7 +75,7 @@ module Gitlab
             end
           end
 
-          Gitlab::Database.main.bulk_insert(IssueAssignee.table_name, assignees) # rubocop:disable Gitlab/BulkInsert
+          ApplicationRecord.legacy_bulk_insert(IssueAssignee.table_name, assignees) # rubocop:disable Gitlab/BulkInsert
         end
       end
     end

lib/gitlab/github_import/importer/label_links_importer.rb

@@ -40,7 +40,7 @@ module Gitlab
             }
           end
 
-          Gitlab::Database.main.bulk_insert(LabelLink.table_name, rows) # rubocop:disable Gitlab/BulkInsert
+          ApplicationRecord.legacy_bulk_insert(LabelLink.table_name, rows) # rubocop:disable Gitlab/BulkInsert
         end
 
         def find_target_id

lib/gitlab/github_import/importer/note_importer.rb

@@ -37,7 +37,7 @@ module Gitlab
           # We're using bulk_insert here so we can bypass any validations and
           # callbacks. Running these would result in a lot of unnecessary SQL
           # queries being executed when importing large projects.
-          Gitlab::Database.main.bulk_insert(Note.table_name, [attributes]) # rubocop:disable Gitlab/BulkInsert
+          ApplicationRecord.legacy_bulk_insert(Note.table_name, [attributes]) # rubocop:disable Gitlab/BulkInsert
         rescue ActiveRecord::InvalidForeignKey
           # It's possible the project and the issue have been deleted since
           # scheduling this job. In this case we'll just skip creating the note.

lib/gitlab/import/database_helpers.rb

@@ -11,8 +11,8 @@ module Gitlab
         # We use bulk_insert here so we can bypass any queries executed by
         # callbacks or validation rules, as doing this wouldn't scale when
         # importing very large projects.
-        result = Gitlab::Database.main # rubocop:disable Gitlab/BulkInsert
-                 .bulk_insert(relation.table_name, [attributes], return_ids: true)
+        result = ApplicationRecord # rubocop:disable Gitlab/BulkInsert
+                 .legacy_bulk_insert(relation.table_name, [attributes], return_ids: true)
 
         result.first
       end

lib/gitlab/language_detection.rb

@@ -18,7 +18,7 @@ module Gitlab
     end
 
     # Newly detected languages, returned in a structure accepted by
-    # Gitlab::Database.main.bulk_insert
+    # ApplicationRecord.legacy_bulk_insert
     def insertions(programming_languages)
       lang_to_id = programming_languages.to_h { |p| [p.name, p.id] }
 

lib/gitlab/sidekiq_middleware/duplicate_jobs/duplicate_job.rb

@@ -208,7 +208,12 @@ module Gitlab
         end
 
         def pg_wal_lsn_diff(connection_name)
-          Gitlab::Database.databases[connection_name].pg_wal_lsn_diff(job_wal_locations[connection_name], existing_wal_locations[connection_name])
+          model = Gitlab::Database.database_base_models[connection_name]
+
+          model.connection.load_balancer.wal_diff(
+            job_wal_locations[connection_name],
+            existing_wal_locations[connection_name]
+          )
         end
 
         def strategy

lib/gitlab/usage_data.rb

@@ -296,9 +296,11 @@ module Gitlab
             version: alt_usage_data(fallback: nil) { Gitlab::CurrentSettings.container_registry_version }
           },
           database: {
-            adapter: alt_usage_data { Gitlab::Database.main.adapter_name },
-            version: alt_usage_data { Gitlab::Database.main.version },
-            pg_system_id: alt_usage_data { Gitlab::Database.main.system_id }
+            # rubocop: disable UsageData/LargeTable
+            adapter: alt_usage_data { ApplicationRecord.database.adapter_name },
+            version: alt_usage_data { ApplicationRecord.database.version },
+            pg_system_id: alt_usage_data { ApplicationRecord.database.system_id }
+            # rubocop: enable UsageData/LargeTable
           },
           mail: {
             smtp_server: alt_usage_data { ActionMailer::Base.smtp_settings[:address] }

lib/tasks/gitlab/db.rake

@@ -182,9 +182,9 @@ namespace :gitlab do
 
     desc 'Enqueue an index for reindexing'
     task :enqueue_reindexing_action, [:index_name, :database] => :environment do |_, args|
-      connection = Gitlab::Database.databases[args.fetch(:database, Gitlab::Database::PRIMARY_DATABASE_NAME)]
+      model = Gitlab::Database.database_base_models[args.fetch(:database, Gitlab::Database::PRIMARY_DATABASE_NAME)]
 
-      Gitlab::Database::SharedModel.using_connection(connection.scope.connection) do
+      Gitlab::Database::SharedModel.using_connection(model.connection) do
         queued_action = Gitlab::Database::PostgresIndex.find(args[:index_name]).queued_reindexing_actions.create!
 
         puts "Queued reindexing action: #{queued_action}"

lib/tasks/gitlab/info.rake

@@ -68,8 +68,8 @@ namespace :gitlab do
       puts "Version:\t#{Gitlab::VERSION}"
       puts "Revision:\t#{Gitlab.revision}"
       puts "Directory:\t#{Rails.root}"
-      puts "DB Adapter:\t#{Gitlab::Database.main.human_adapter_name}"
-      puts "DB Version:\t#{Gitlab::Database.main.version}"
+      puts "DB Adapter:\t#{ApplicationRecord.database.human_adapter_name}"
+      puts "DB Version:\t#{ApplicationRecord.database.version}"
       puts "URL:\t\t#{Gitlab.config.gitlab.url}"
       puts "HTTP Clone URL:\t#{http_clone_url}"
       puts "SSH Clone URL:\t#{ssh_clone_url}"

lib/tasks/gitlab/storage.rake

@@ -170,7 +170,7 @@ namespace :gitlab do
       inverval = (ENV['MAX_DATABASE_CONNECTION_CHECK_INTERVAL'] || 10).to_f
 
       attempts.to_i.times do
-        unless Gitlab::Database.main.exists?
+        unless ApplicationRecord.database.exists?
           puts "Waiting until database is ready before continuing...".color(:yellow)
           sleep inverval
         end

rubocop/cop/gitlab/bulk_insert.rb

@@ -3,13 +3,13 @@
 module RuboCop
   module Cop
     module Gitlab
-      # Cop that disallows the use of `Gitlab::Database.main.bulk_insert`, in favour of using
+      # Cop that disallows the use of `legacy_bulk_insert`, in favour of using
       # the `BulkInsertSafe` module.
       class BulkInsert < RuboCop::Cop::Cop
-        MSG = 'Use the `BulkInsertSafe` concern, instead of using `Gitlab::Database.main.bulk_insert`. See https://docs.gitlab.com/ee/development/insert_into_tables_in_batches.html'
+        MSG = 'Use the `BulkInsertSafe` concern, instead of using `LegacyBulkInsert.bulk_insert`. See https://docs.gitlab.com/ee/development/insert_into_tables_in_batches.html'
 
         def_node_matcher :raw_union?, <<~PATTERN
-          (send (send (const (const _ :Gitlab) :Database) :main) :bulk_insert ...)
+          (send _ :legacy_bulk_insert ...)
         PATTERN
 
         def on_send(node)

spec/factories/design_management/designs.rb

@@ -39,7 +39,7 @@ FactoryBot.define do
           sha = commit_version[action]
           version = DesignManagement::Version.new(sha: sha, issue: issue, author: evaluator.author)
           version.save!(validate: false) # We need it to have an ID, validate later
-          Gitlab::Database.main.bulk_insert(dv_table_name, [action.row_attrs(version)]) # rubocop:disable Gitlab/BulkInsert
+          ApplicationRecord.legacy_bulk_insert(dv_table_name, [action.row_attrs(version)]) # rubocop:disable Gitlab/BulkInsert
         end
 
         # always a creation

spec/initializers/database_config_spec.rb

@@ -8,11 +8,11 @@ RSpec.describe 'Database config initializer', :reestablished_active_record_base
   end
 
   it 'retains the correct database name for the connection' do
-    previous_db_name = Gitlab::Database.main.scope.connection.pool.db_config.name
+    previous_db_name = ApplicationRecord.connection.pool.db_config.name
 
     subject
 
-    expect(Gitlab::Database.main.scope.connection.pool.db_config.name).to eq(previous_db_name)
+    expect(ApplicationRecord.connection.pool.db_config.name).to eq(previous_db_name)
   end
 
   it 'does not overwrite custom pool settings' do

spec/lib/feature/gitaly_spec.rb

@@ -78,7 +78,9 @@ RSpec.describe Feature::Gitaly do
 
     context 'when table does not exist' do
       before do
-        allow(::Gitlab::Database.main).to receive(:cached_table_exists?).and_return(false)
+        allow(Feature::FlipperFeature.database)
+          .to receive(:cached_table_exists?)
+          .and_return(false)
       end
 
       it 'returns an empty Hash' do

spec/lib/feature_spec.rb

@@ -314,7 +314,7 @@ RSpec.describe Feature, stub_feature_flags: false do
 
             context 'when database exists' do
               before do
-                allow(Gitlab::Database.main).to receive(:exists?).and_return(true)
+                allow(ApplicationRecord.database).to receive(:exists?).and_return(true)
               end
 
               it 'checks the persisted status and returns false' do
@@ -326,7 +326,7 @@ RSpec.describe Feature, stub_feature_flags: false do
 
             context 'when database does not exist' do
               before do
-                allow(Gitlab::Database.main).to receive(:exists?).and_return(false)
+                allow(ApplicationRecord.database).to receive(:exists?).and_return(false)
               end
 
               it 'returns false without checking the status in the database' do

spec/lib/gitlab/background_migration/job_coordinator_spec.rb

@@ -42,7 +42,7 @@ RSpec.describe Gitlab::BackgroundMigration::JobCoordinator do
   describe '#with_shared_connection' do
     it 'yields to the block after properly configuring SharedModel' do
       expect(Gitlab::Database::SharedModel).to receive(:using_connection)
-        .with(Gitlab::Database.main.scope.connection).and_yield
+        .with(ActiveRecord::Base.connection).and_yield
 
       expect { |b| coordinator.with_shared_connection(&b) }.to yield_with_no_args
     end

spec/lib/gitlab/config_checker/external_database_checker_spec.rb

@@ -8,7 +8,7 @@ RSpec.describe Gitlab::ConfigChecker::ExternalDatabaseChecker do
 
     context 'when database meets minimum supported version' do
       before do
-        allow(Gitlab::Database.main).to receive(:postgresql_minimum_supported_version?).and_return(true)
+        allow(ApplicationRecord.database).to receive(:postgresql_minimum_supported_version?).and_return(true)
       end
 
       it { is_expected.to be_empty }
@@ -16,7 +16,7 @@ RSpec.describe Gitlab::ConfigChecker::ExternalDatabaseChecker do
 
     context 'when database does not meet minimum supported version' do
       before do
-        allow(Gitlab::Database.main).to receive(:postgresql_minimum_supported_version?).and_return(false)
+        allow(ApplicationRecord.database).to receive(:postgresql_minimum_supported_version?).and_return(false)
       end
 
       let(:notice_deprecated_database) do
@@ -26,7 +26,7 @@ RSpec.describe Gitlab::ConfigChecker::ExternalDatabaseChecker do
                      '%{pg_version_minimum} is required for this version of GitLab. ' \
                      'Please upgrade your environment to a supported PostgreSQL version, ' \
                      'see %{pg_requirements_url} for details.') % {
-                                                                    pg_version_current: Gitlab::Database.main.version,
+                                                                    pg_version_current: ApplicationRecord.database.version,
                                                                     pg_version_minimum: Gitlab::Database::MINIMUM_POSTGRES_VERSION,
                                                                     pg_requirements_url: '<a href="https://docs.gitlab.com/ee/install/requirements.html#database">database requirements</a>'
                                                                   }

spec/lib/gitlab/database/each_database_spec.rb

@@ -3,9 +3,9 @@
 require 'spec_helper'
 
 RSpec.describe Gitlab::Database::EachDatabase do
-  describe '.each_database' do
+  describe '.each_database_connection' do
     let(:expected_connections) do
-      Gitlab::Database.databases.map { |name, wrapper| [wrapper.scope.connection, name] }
+      Gitlab::Database.database_base_models.map { |name, model| [model.connection, name] }
     end
 
     it 'yields each connection after connecting SharedModel' do

spec/lib/gitlab/database/load_balancing/load_balancer_spec.rb

@@ -542,4 +542,17 @@ RSpec.describe Gitlab::Database::LoadBalancing::LoadBalancer, :request_store do
       end
     end
   end
+
+  describe '#wal_diff' do
+    it 'returns the diff between two write locations' do
+      loc1 = lb.send(:get_write_location, lb.pool.connection)
+
+      create(:user) # This ensures we get a new WAL location
+
+      loc2 = lb.send(:get_write_location, lb.pool.connection)
+      diff = lb.wal_diff(loc2, loc1)
+
+      expect(diff).to be_positive
+    end
+  end
 end

spec/lib/gitlab/database/partitioning/partition_manager_spec.rb

@@ -195,7 +195,7 @@ RSpec.describe Gitlab::Database::Partitioning::PartitionManager do
     end
 
     # Postgres 11 does not support foreign keys to partitioned tables
-    if Gitlab::Database.main.version.to_f >= 12
+    if ApplicationRecord.database.version.to_f >= 12
       context 'when the model is the target of a foreign key' do
         before do
           connection.execute(<<~SQL)

spec/lib/gitlab/database_spec.rb

@@ -15,13 +15,6 @@ RSpec.describe Gitlab::Database do
     end
   end
 
-  describe '.databases' do
-    it 'stores connections as a HashWithIndifferentAccess' do
-      expect(described_class.databases.has_key?('main')).to be true
-      expect(described_class.databases.has_key?(:main)).to be true
-    end
-  end
-
   describe '.default_pool_size' do
     before do
       allow(Gitlab::Runtime).to receive(:max_threads).and_return(7)
@@ -112,18 +105,30 @@ RSpec.describe Gitlab::Database do
   end
 
   describe '.check_postgres_version_and_print_warning' do
+    let(:reflect) { instance_spy(Gitlab::Database::Reflection) }
+
     subject { described_class.check_postgres_version_and_print_warning }
 
+    before do
+      allow(Gitlab::Database::Reflection)
+        .to receive(:new)
+        .and_return(reflect)
+    end
+
     it 'prints a warning if not compliant with minimum postgres version' do
-      allow(described_class.main).to receive(:postgresql_minimum_supported_version?).and_return(false)
+      allow(reflect).to receive(:postgresql_minimum_supported_version?).and_return(false)
 
-      expect(Kernel).to receive(:warn).with(/You are using PostgreSQL/)
+      expect(Kernel)
+        .to receive(:warn)
+        .with(/You are using PostgreSQL/)
+        .exactly(Gitlab::Database.database_base_models.length)
+        .times
 
       subject
     end
 
     it 'doesnt print a warning if compliant with minimum postgres version' do
-      allow(described_class.main).to receive(:postgresql_minimum_supported_version?).and_return(true)
+      allow(reflect).to receive(:postgresql_minimum_supported_version?).and_return(true)
 
       expect(Kernel).not_to receive(:warn).with(/You are using PostgreSQL/)
 
@@ -131,7 +136,7 @@ RSpec.describe Gitlab::Database do
     end
 
     it 'doesnt print a warning in Rails runner environment' do
-      allow(described_class.main).to receive(:postgresql_minimum_supported_version?).and_return(false)
+      allow(reflect).to receive(:postgresql_minimum_supported_version?).and_return(false)
       allow(Gitlab::Runtime).to receive(:rails_runner?).and_return(true)
 
       expect(Kernel).not_to receive(:warn).with(/You are using PostgreSQL/)
@@ -140,13 +145,13 @@ RSpec.describe Gitlab::Database do
     end
 
     it 'ignores ActiveRecord errors' do
-      allow(described_class.main).to receive(:postgresql_minimum_supported_version?).and_raise(ActiveRecord::ActiveRecordError)
+      allow(reflect).to receive(:postgresql_minimum_supported_version?).and_raise(ActiveRecord::ActiveRecordError)
 
       expect { subject }.not_to raise_error
     end
 
     it 'ignores Postgres errors' do
-      allow(described_class.main).to receive(:postgresql_minimum_supported_version?).and_raise(PG::Error)
+      allow(reflect).to receive(:postgresql_minimum_supported_version?).and_raise(PG::Error)
 
       expect { subject }.not_to raise_error
     end

spec/lib/gitlab/github_import/bulk_importing_spec.rb

@@ -116,13 +116,13 @@ RSpec.describe Gitlab::GithubImport::BulkImporting do
           value: 5
         )
 
-      expect(Gitlab::Database.main)
-        .to receive(:bulk_insert)
+      expect(ApplicationRecord)
+        .to receive(:legacy_bulk_insert)
         .ordered
         .with('kittens', rows.first(5))
 
-      expect(Gitlab::Database.main)
-        .to receive(:bulk_insert)
+      expect(ApplicationRecord)
+        .to receive(:legacy_bulk_insert)
         .ordered
         .with('kittens', rows.last(5))
 

spec/lib/gitlab/github_import/importer/diff_note_importer_spec.rb

@@ -82,8 +82,8 @@ RSpec.describe Gitlab::GithubImport::Importer::DiffNoteImporter, :aggregate_fail
     it 'does not import the note when a foreign key error is raised' do
       stub_user_finder(project.creator_id, false)
 
-      expect(Gitlab::Database.main)
-        .to receive(:bulk_insert)
+      expect(ApplicationRecord)
+        .to receive(:legacy_bulk_insert)
         .and_raise(ActiveRecord::InvalidForeignKey, 'invalid foreign key')
 
       expect { subject.execute }
@@ -94,6 +94,8 @@ RSpec.describe Gitlab::GithubImport::Importer::DiffNoteImporter, :aggregate_fail
   describe '#execute' do
     context 'when the merge request no longer exists' do
       it 'does not import anything' do
+        expect(ApplicationRecord).not_to receive(:legacy_bulk_insert)
+
         expect { subject.execute }
           .to not_change(DiffNote, :count)
           .and not_change(LegacyDiffNote, :count)

spec/lib/gitlab/github_import/importer/issue_importer_spec.rb

@@ -190,8 +190,8 @@ RSpec.describe Gitlab::GithubImport::Importer::IssueImporter, :clean_gitlab_redi
         .with(issue.assignees[1])
         .and_return(5)
 
-      expect(Gitlab::Database.main)
-        .to receive(:bulk_insert)
+      expect(ApplicationRecord)
+        .to receive(:legacy_bulk_insert)
         .with(
           IssueAssignee.table_name,
           [{ issue_id: 1, user_id: 4 }, { issue_id: 1, user_id: 5 }]

spec/lib/gitlab/github_import/importer/label_links_importer_spec.rb

@@ -39,8 +39,8 @@ RSpec.describe Gitlab::GithubImport::Importer::LabelLinksImporter do
         .and_return(1)
 
       freeze_time do
-        expect(Gitlab::Database.main)
-          .to receive(:bulk_insert)
+        expect(ApplicationRecord)
+          .to receive(:legacy_bulk_insert)
           .with(
             LabelLink.table_name,
             [
@@ -64,8 +64,8 @@ RSpec.describe Gitlab::GithubImport::Importer::LabelLinksImporter do
         .with('bug')
         .and_return(nil)
 
-      expect(Gitlab::Database.main)
-        .to receive(:bulk_insert)
+      expect(ApplicationRecord)
+        .to receive(:legacy_bulk_insert)
         .with(LabelLink.table_name, [])
 
       importer.create_labels

spec/lib/gitlab/github_import/importer/note_importer_spec.rb

@@ -41,8 +41,8 @@ RSpec.describe Gitlab::GithubImport::Importer::NoteImporter do
             .with(github_note)
             .and_return([user.id, true])
 
-          expect(Gitlab::Database.main)
-            .to receive(:bulk_insert)
+          expect(ApplicationRecord)
+            .to receive(:legacy_bulk_insert)
             .with(
               Note.table_name,
               [
@@ -71,8 +71,8 @@ RSpec.describe Gitlab::GithubImport::Importer::NoteImporter do
             .with(github_note)
             .and_return([project.creator_id, false])
 
-          expect(Gitlab::Database.main)
-            .to receive(:bulk_insert)
+          expect(ApplicationRecord)
+            .to receive(:legacy_bulk_insert)
             .with(
               Note.table_name,
               [
@@ -115,7 +115,7 @@ RSpec.describe Gitlab::GithubImport::Importer::NoteImporter do
 
     context 'when the noteable does not exist' do
       it 'does not import the note' do
-        expect(Gitlab::Database.main).not_to receive(:bulk_insert)
+        expect(ApplicationRecord).not_to receive(:legacy_bulk_insert)
 
         importer.execute
       end
@@ -134,8 +134,8 @@ RSpec.describe Gitlab::GithubImport::Importer::NoteImporter do
           .with(github_note)
           .and_return([user.id, true])
 
-        expect(Gitlab::Database.main)
-          .to receive(:bulk_insert)
+        expect(ApplicationRecord)
+          .to receive(:legacy_bulk_insert)
           .and_raise(ActiveRecord::InvalidForeignKey, 'invalid foreign key')
 
         expect { importer.execute }.not_to raise_error

spec/lib/gitlab/import/database_helpers_spec.rb

@@ -16,8 +16,8 @@ RSpec.describe Gitlab::Import::DatabaseHelpers do
     let(:project) { create(:project) }
 
     it 'returns the ID returned by the query' do
-      expect(Gitlab::Database.main)
-        .to receive(:bulk_insert)
+      expect(ApplicationRecord)
+        .to receive(:legacy_bulk_insert)
         .with(Issue.table_name, [attributes], return_ids: true)
         .and_return([10])
 

spec/lib/gitlab/metrics/samplers/database_sampler_spec.rb

@@ -18,8 +18,8 @@ RSpec.describe Gitlab::Metrics::Samplers::DatabaseSampler do
       let(:labels) do
         {
           class: 'ActiveRecord::Base',
-          host: Gitlab::Database.main.config['host'],
-          port: Gitlab::Database.main.config['port']
+          host: ApplicationRecord.database.config['host'],
+          port: ApplicationRecord.database.config['port']
         }
       end
 

spec/lib/gitlab/usage/metrics/instrumentations/generic_metric_spec.rb

@@ -7,18 +7,18 @@ RSpec.describe Gitlab::Usage::Metrics::Instrumentations::GenericMetric do
     subject do
       Class.new(described_class) do
         fallback(custom_fallback)
-        value { Gitlab::Database.main.version }
+        value { ApplicationRecord.database.version }
       end.new(time_frame: 'none')
     end
 
     describe '#value' do
       it 'gives the correct value' do
-        expect(subject.value).to eq(Gitlab::Database.main.version)
+        expect(subject.value).to eq(ApplicationRecord.database.version)
       end
 
       context 'when raising an exception' do
         it 'return the custom fallback' do
-          expect(Gitlab::Database.main).to receive(:version).and_raise('Error')
+          expect(ApplicationRecord.database).to receive(:version).and_raise('Error')
           expect(subject.value).to eq(custom_fallback)
         end
       end
@@ -28,18 +28,18 @@ RSpec.describe Gitlab::Usage::Metrics::Instrumentations::GenericMetric do
   context 'with default fallback' do
     subject do
       Class.new(described_class) do
-        value { Gitlab::Database.main.version }
+        value { ApplicationRecord.database.version }
       end.new(time_frame: 'none')
     end
 
     describe '#value' do
       it 'gives the correct value' do
-        expect(subject.value).to eq(Gitlab::Database.main.version )
+        expect(subject.value).to eq(ApplicationRecord.database.version )
       end
 
       context 'when raising an exception' do
         it 'return the default fallback' do
-          expect(Gitlab::Database.main).to receive(:version).and_raise('Error')
+          expect(ApplicationRecord.database).to receive(:version).and_raise('Error')
           expect(subject.value).to eq(described_class::FALLBACK)
         end
       end

spec/lib/gitlab/usage_data_spec.rb

@@ -978,9 +978,9 @@ RSpec.describe Gitlab::UsageData, :aggregate_failures do
         expect(subject[:gitlab_pages][:enabled]).to eq(Gitlab.config.pages.enabled)
         expect(subject[:gitlab_pages][:version]).to eq(Gitlab::Pages::VERSION)
         expect(subject[:git][:version]).to eq(Gitlab::Git.version)
-        expect(subject[:database][:adapter]).to eq(Gitlab::Database.main.adapter_name)
-        expect(subject[:database][:version]).to eq(Gitlab::Database.main.version)
-        expect(subject[:database][:pg_system_id]).to eq(Gitlab::Database.main.system_id)
+        expect(subject[:database][:adapter]).to eq(ApplicationRecord.database.adapter_name)
+        expect(subject[:database][:version]).to eq(ApplicationRecord.database.version)
+        expect(subject[:database][:pg_system_id]).to eq(ApplicationRecord.database.system_id)
         expect(subject[:mail][:smtp_server]).to eq(ActionMailer::Base.smtp_settings[:address])
         expect(subject[:gitaly][:version]).to be_present
         expect(subject[:gitaly][:servers]).to be >= 1

spec/models/ci/build_spec.rb

@@ -3806,7 +3806,7 @@ RSpec.describe Ci::Build do
 
       it 'ensures that it is not run in database transaction' do
         expect(job.pipeline.persistent_ref).to receive(:create) do
-          expect(Gitlab::Database.main).not_to be_inside_transaction
+          expect(ApplicationRecord).not_to be_inside_transaction
         end
 
         run_job_without_exception

spec/models/concerns/bulk_insert_safe_spec.rb

@@ -182,7 +182,7 @@ RSpec.describe BulkInsertSafe do
       context 'with returns option set' do
         let(:items) { bulk_insert_item_class.valid_list(1) }
 
-        subject(:bulk_insert) { bulk_insert_item_class.bulk_insert!(items, returns: returns) }
+        subject(:legacy_bulk_insert) { bulk_insert_item_class.bulk_insert!(items, returns: returns) }
 
         context 'when is set to :ids' do
           let(:returns) { :ids }

spec/models/concerns/database_reflection_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe DatabaseReflection do
+  describe '.reflect' do
+    it 'returns a Reflection instance' do
+      expect(User.database).to be_an_instance_of(Gitlab::Database::Reflection)
+    end
+
+    it 'memoizes the result' do
+      instance1 = User.database
+      instance2 = User.database
+
+      expect(instance1).to equal(instance2)
+    end
+  end
+end