Swap FK merge_trains to ci_pipelines for LFK

Swaps FK for merge_trains.pipeline_id to ci_pipelines

Changelog: changed

db/post_migrate/20220119153706_remove_ci_pipelines_merge_trains_pipeline_id_fk.rb

+# frozen_string_literal: true
+
+class RemoveCiPipelinesMergeTrainsPipelineIdFk < Gitlab::Database::Migration[1.0]
+  disable_ddl_transaction!
+
+  def up
+    with_lock_retries do
+      execute('LOCK ci_pipelines, merge_trains IN ACCESS EXCLUSIVE MODE')
+
+      remove_foreign_key_if_exists(:merge_trains, :ci_pipelines, name: "fk_rails_f90820cb08")
+    end
+  end
+
+  def down
+    add_concurrent_foreign_key(:merge_trains, :ci_pipelines, name: "fk_rails_f90820cb08", column: :pipeline_id, target_column: :id, on_delete: :nullify)
+  end
+end

db/schema_migrations/20220119153706

+f87eec3c61cfc9dc860fe77af1fbe98c1ea2a9b0dc1b2dbdaaee4276b518af73
\ No newline at end of file

db/structure.sql

@@ -31529,9 +31529,6 @@ ALTER TABLE ONLY security_trainings
 ALTER TABLE ONLY merge_requests_closing_issues
     ADD CONSTRAINT fk_rails_f8540692be FOREIGN KEY (issue_id) REFERENCES issues(id) ON DELETE CASCADE;
 
-ALTER TABLE ONLY merge_trains
-    ADD CONSTRAINT fk_rails_f90820cb08 FOREIGN KEY (pipeline_id) REFERENCES ci_pipelines(id) ON DELETE SET NULL;
-
 ALTER TABLE ONLY banned_users
     ADD CONSTRAINT fk_rails_fa5bb598e5 FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE;
 

ee/spec/models/merge_train_spec.rb

@@ -656,4 +656,11 @@ RSpec.describe MergeTrain do
       source_project: source_project,
       status: MergeTrain.state_machines[:status].states[status].value)
   end
+
+  context 'loose foreign key on merge_trains.pipeline_id' do
+    it_behaves_like 'cleanup by a loose foreign key' do
+      let!(:parent) { create(:ci_pipeline) }
+      let!(:model) { create(:merge_train, pipeline: parent) }
+    end
+  end
 end

lib/gitlab/database/gitlab_loose_foreign_keys.yml

@@ -134,6 +134,10 @@ ci_pipeline_schedules:
   - table: users
     column: owner_id
     on_delete: async_nullify
+merge_trains:
+  - table: ci_pipelines
+    column: pipeline_id
+    on_delete: async_nullify
 ci_group_variables:
   - table: namespaces
     column: group_id

spec/lib/gitlab/database/no_cross_db_foreign_keys_spec.rb

@@ -44,7 +44,6 @@ RSpec.describe 'cross-database foreign keys' do
       dast_profiles_pipelines.ci_pipeline_id
       dast_site_profiles_pipelines.ci_pipeline_id
       external_pull_requests.project_id
-      merge_trains.pipeline_id
       vulnerability_feedback.pipeline_id
       vulnerability_occurrence_pipelines.pipeline_id
       vulnerability_statistics.latest_pipeline_id