Fix use of ActiveRecord::Base in CurrentSettings

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/350651

.rubocop_todo/database/multiple_databases.yml

@@ -11,7 +11,6 @@ Database/MultipleDatabases:
   - ee/spec/services/ee/merge_requests/update_service_spec.rb
   - lib/backup/database.rb
   - lib/backup/manager.rb
-  - lib/gitlab/current_settings.rb
   - lib/gitlab/database/load_balancing/load_balancer.rb
   - lib/gitlab/database/load_balancing.rb
   - lib/gitlab/database/load_balancing/sticking.rb
@@ -29,7 +28,6 @@ Database/MultipleDatabases:
   - spec/db/schema_spec.rb
   - spec/initializers/database_config_spec.rb
   - spec/lib/backup/manager_spec.rb
-  - spec/lib/gitlab/current_settings_spec.rb
   - spec/lib/gitlab/database_spec.rb
   - spec/lib/gitlab/metrics/subscribers/active_record_spec.rb
   - spec/lib/gitlab/profiler_spec.rb

lib/gitlab/current_settings.rb

@@ -62,7 +62,7 @@ module Gitlab
         # need to be added to the application settings. To prevent Rake tasks
         # and other callers from failing, use any loaded settings and return
         # defaults for missing columns.
-        if Gitlab::Runtime.rake? && ActiveRecord::Base.connection.migration_context.needs_migration?
+        if Gitlab::Runtime.rake? && ::ApplicationSetting.connection.migration_context.needs_migration?
           db_attributes = current_settings&.attributes || {}
           fake_application_settings(db_attributes)
         elsif current_settings.present?
@@ -82,7 +82,7 @@ module Gitlab
 
       def connect_to_db?
         # When the DBMS is not available, an exception (e.g. PG::ConnectionBad) is raised
-        active_db_connection = ActiveRecord::Base.connection.active? rescue false
+        active_db_connection = ::ApplicationSetting.connection.active? rescue false
 
         active_db_connection &&
           ApplicationSetting.database.cached_table_exists?

spec/lib/gitlab/current_settings_spec.rb

@@ -118,7 +118,7 @@ RSpec.describe Gitlab::CurrentSettings do
         allow(Gitlab::Runtime).to receive(:rake?).and_return(true)
         # For some reason, `allow(described_class).to receive(:connect_to_db?).and_return(false)` causes issues
         # during the initialization phase of the test suite, so instead let's mock the internals of it
-        allow(ActiveRecord::Base.connection).to receive(:active?).and_return(false)
+        allow(ApplicationSetting.connection).to receive(:active?).and_return(false)
       end
 
       context 'and no settings in cache' do
@@ -150,8 +150,8 @@ RSpec.describe Gitlab::CurrentSettings do
         it 'fetches the settings from cache' do
           # For some reason, `allow(described_class).to receive(:connect_to_db?).and_return(true)` causes issues
           # during the initialization phase of the test suite, so instead let's mock the internals of it
-          expect(ActiveRecord::Base.connection).not_to receive(:active?)
-          expect(ActiveRecord::Base.connection).not_to receive(:cached_table_exists?)
+          expect(ApplicationSetting.connection).not_to receive(:active?)
+          expect(ApplicationSetting.connection).not_to receive(:cached_table_exists?)
           expect_any_instance_of(ActiveRecord::MigrationContext).not_to receive(:needs_migration?)
           expect(ActiveRecord::QueryRecorder.new { described_class.current_application_settings }.count).to eq(0)
         end
@@ -159,8 +159,8 @@ RSpec.describe Gitlab::CurrentSettings do
 
       context 'and no settings in cache' do
         before do
-          allow(ActiveRecord::Base.connection).to receive(:active?).and_return(true)
-          allow(ActiveRecord::Base.connection).to receive(:cached_table_exists?).with('application_settings').and_return(true)
+          allow(ApplicationSetting.connection).to receive(:active?).and_return(true)
+          allow(ApplicationSetting.connection).to receive(:cached_table_exists?).with('application_settings').and_return(true)
         end
 
         context 'with RequestStore enabled', :request_store do
@@ -181,7 +181,7 @@ RSpec.describe Gitlab::CurrentSettings do
 
         context 'when ApplicationSettings does not have a primary key' do
           before do
-            allow(ActiveRecord::Base.connection).to receive(:primary_key).with('application_settings').and_return(nil)
+            allow(ApplicationSetting.connection).to receive(:primary_key).with('application_settings').and_return(nil)
           end
 
           it 'raises an exception if ApplicationSettings does not have a primary key' do