Remove FK security_scans.build_id as we have LFK

In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/77908 we
already added a ["loose foreign key"](
https://docs.gitlab.com/ee/development/database/loose_foreign_keys.html)
to replace this foreign key. Now that this has been running fine in
production we should be OK to remove the foreign key altogether.

This was added in a post-deployment migration as we want to ensure that
they have the correct code running with the loose foreign key in place
before we remove the foreign key to ensure that there isn't any lost
deletes.

Changelog: other

db/post_migrate/20220111002756_remove_security_scans_build_id_fk.rb

+# frozen_string_literal: true
+
+class RemoveSecurityScansBuildIdFk < Gitlab::Database::Migration[1.0]
+  disable_ddl_transaction!
+
+  CONSTRAINT_NAME = 'fk_rails_4ef1e6b4c6'
+
+  def up
+    with_lock_retries do
+      execute('LOCK ci_builds, security_scans IN ACCESS EXCLUSIVE MODE')
+      remove_foreign_key_if_exists(:security_scans, :ci_builds, name: CONSTRAINT_NAME)
+    end
+  end
+
+  def down
+    add_concurrent_foreign_key(:security_scans, :ci_builds, column: :build_id, on_delete: :cascade, name: CONSTRAINT_NAME)
+  end
+end

db/schema_migrations/20220111002756

+34759cbf09171f6057b87af791f5e9f3045ac5e06558147436ba32e276f40a19
\ No newline at end of file

db/structure.sql

@@ -30496,9 +30496,6 @@ ALTER TABLE ONLY geo_repository_renamed_events
 ALTER TABLE ONLY aws_roles
     ADD CONSTRAINT fk_rails_4ed56f4720 FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE;
 
-ALTER TABLE ONLY security_scans
-    ADD CONSTRAINT fk_rails_4ef1e6b4c6 FOREIGN KEY (build_id) REFERENCES ci_builds(id) ON DELETE CASCADE;
-
 ALTER TABLE ONLY packages_debian_publications
     ADD CONSTRAINT fk_rails_4fc8ebd03e FOREIGN KEY (distribution_id) REFERENCES packages_debian_project_distributions(id) ON DELETE CASCADE;
 

ee/spec/models/security/scan_spec.rb

@@ -3,6 +3,11 @@
 require 'spec_helper'
 
 RSpec.describe Security::Scan do
+  it_behaves_like 'cleanup by a loose foreign key' do
+    let!(:model) { create(:security_scan) }
+    let(:parent) { model.build }
+  end
+
   describe 'associations' do
     it { is_expected.to belong_to(:build) }
     it { is_expected.to belong_to(:project) }

spec/support/shared_examples/loose_foreign_keys/have_loose_foreign_key.rb

@@ -55,8 +55,13 @@ RSpec.shared_examples 'cleanup by a loose foreign key' do
   end
 
   def find_model
-    primary_key = model.class.primary_key.to_sym
-    model.class.find_by(primary_key => model.public_send(primary_key))
+    query = model.class
+    # handle composite primary keys
+    connection = model.class.connection
+    connection.primary_keys(model.class.table_name).each do |primary_key|
+      query = query.where(primary_key => model.public_send(primary_key))
+    end
+    query.first
   end
 
   it 'cleans up (delete or nullify) the model' do