Add back user_mentions background migration

config/feature_flags/development/migrate_user_mentions.yml

+---
+# While this FF is enabled by default, we want to keep it in place for now as there are still related migrations that
+# have not been run in production yet. For additional info, see https://gitlab.com/gitlab-org/gitlab/-/issues/21801
+# Discussion and removal MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/60322
+name: migrate_user_mentions
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/34378
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/231175
+milestone: '13.3'
+type: development
+group: group::project management
+default_enabled: true

lib/gitlab/background_migration/user_mentions/create_resource_user_mention.rb

+# frozen_string_literal: true
+# rubocop:disable Style/Documentation
+
+module Gitlab
+  module BackgroundMigration
+    module UserMentions
+      class CreateResourceUserMention
+        # Resources that have mentions to be migrated:
+        # issue, merge_request, epic, commit, snippet, design
+
+        BULK_INSERT_SIZE = 1_000
+        ISOLATION_MODULE = 'Gitlab::BackgroundMigration::UserMentions::Models'
+
+        def perform(resource_model, join, conditions, with_notes, start_id, end_id)
+          return unless Feature.enabled?(:migrate_user_mentions, default_enabled: true)
+
+          resource_model = "#{ISOLATION_MODULE}::#{resource_model}".constantize if resource_model.is_a?(String)
+          model = with_notes ? Gitlab::BackgroundMigration::UserMentions::Models::Note : resource_model
+          resource_user_mention_model = resource_model.user_mention_model
+
+          records = model.joins(join).where(conditions).where(id: start_id..end_id)
+
+          records.each_batch(of: BULK_INSERT_SIZE) do |records|
+            mentions = []
+            records.each do |record|
+              mention_record = record.build_mention_values(resource_user_mention_model.resource_foreign_key)
+              mentions << mention_record unless mention_record.blank?
+            end
+
+            resource_user_mention_model.insert_all(mentions) unless mentions.empty?
+          end
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/background_migration/user_mentions/create_resource_user_mention_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::BackgroundMigration::UserMentions::CreateResourceUserMention, schema: 20181228175414 do
+  context 'checks no_quote_columns' do
+    it 'has correct no_quote_columns' do
+      expect(Gitlab::BackgroundMigration::UserMentions::Models::MergeRequest.no_quote_columns).to match([:note_id, :merge_request_id])
+    end
+
+    it 'commit has correct no_quote_columns' do
+      expect(Gitlab::BackgroundMigration::UserMentions::Models::Commit.no_quote_columns).to match([:note_id])
+    end
+  end
+end