Bulk migrate multiple project's MR

This speeds up migration, as previously each project has its own job,
now they can be in a batch, avoiding 2 minutes wait interval per job.

ee/lib/gitlab/background_migration/migrate_approver_to_approval_rules.rb

@@ -53,7 +53,6 @@ module Gitlab
 
       class MergeRequest < ActiveRecord::Base
         self.table_name = 'merge_requests'
-        include ::EachBatch
 
         belongs_to :target_project, class_name: "Project"
         has_many :approval_rules, class_name: 'ApprovalMergeRequestRule'
@@ -74,7 +73,6 @@ module Gitlab
       class Project < ActiveRecord::Base
         self.table_name = 'projects'
 
-        has_many :merge_requests, foreign_key: 'target_project_id', inverse_of: :target_project
         has_many :approval_rules, class_name: 'ApprovalProjectRule'
 
         def approvers
@@ -124,8 +122,6 @@ module Gitlab
         ActiveRecord::Base.transaction do
           sync_rule
         end
-
-        schedule_to_migrate_merge_requests(target)
       end
 
       def sync_rule
@@ -140,14 +136,6 @@ module Gitlab
         rule
       end
 
-      def schedule_to_migrate_merge_requests(project)
-        jobs = []
-        project.merge_requests.each_batch do |scope, _|
-          jobs << ['MigrateApproverToApprovalRulesInBatch', ['MergeRequest', scope.pluck(:id)]]
-        end
-        BackgroundMigrationWorker.bulk_perform_async(jobs)
-      end
-
       def target
         strong_memoize(:target) do
           case @target_type

ee/lib/gitlab/background_migration/migrate_approver_to_approval_rules_in_batch.rb

@@ -3,10 +3,27 @@
 module Gitlab
   module BackgroundMigration
     class MigrateApproverToApprovalRulesInBatch
+      class MergeRequest < ActiveRecord::Base
+        self.table_name = 'merge_requests'
+        include ::EachBatch
+      end
+
       def perform(target_type, target_ids)
         target_ids.each do |target_id|
           MigrateApproverToApprovalRules.new.perform(target_type, target_id)
         end
+
+        schedule_to_migrate_merge_requests(target_ids) if target_type == 'Project'
+      end
+
+      private
+
+      def schedule_to_migrate_merge_requests(project_ids)
+        jobs = []
+        MergeRequest.where(target_project_id: project_ids).each_batch do |scope, _|
+          jobs << [self.class.name, ['MergeRequest', scope.pluck(:id)]]
+        end
+        BackgroundMigrationWorker.bulk_perform_async(jobs) unless jobs.empty?
       end
     end
   end

ee/spec/lib/gitlab/background_migration/migrate_approver_to_approval_rules_in_batch_spec.rb

@@ -6,12 +6,36 @@ describe Gitlab::BackgroundMigration::MigrateApproverToApprovalRulesInBatch do
   context 'when there is no more MigrateApproverToApprovalRules jobs' do
     let(:job) { double(:job) }
 
-    it 'enables feature' do
+    it 'migrates individual target' do
       allow(Gitlab::BackgroundMigration::MigrateApproverToApprovalRules).to receive(:new).and_return(job)
 
       expect(job).to receive(:perform).exactly(3).times
 
       described_class.new.perform('Foo', [1, 2, 3])
     end
+
+    context 'when targets are projects' do
+      let(:projects) { create_list(:project, 3) }
+
+      context 'when projects contain merge requests' do
+        it 'schedules migrations for merge requests' do
+          merge_requests = projects.flat_map do |project|
+            create(:merge_request, source_project: project, target_project: project)
+          end
+
+          expect(BackgroundMigrationWorker).to receive(:bulk_perform_async).with([[described_class.name, ["MergeRequest", merge_requests.map(&:id)]]])
+
+          described_class.new.perform('Project', projects.map(&:id))
+        end
+      end
+
+      context 'when merge request do not exist' do
+        it 'does nothing' do
+          expect(BackgroundMigrationWorker).not_to receive(:bulk_perform_async)
+
+          described_class.new.perform('Project', projects.map(&:id))
+        end
+      end
+    end
   end
 end

ee/spec/lib/gitlab/background_migration/migrate_approver_to_approval_rules_spec.rb

@@ -178,16 +178,6 @@ describe Gitlab::BackgroundMigration::MigrateApproverToApprovalRules do
       let(:approval_rule) { create(:approval_project_rule, project: target) }
 
       it_behaves_like 'sync approval member'
-
-      context 'when project contains some merge requests' do
-        let!(:merge_request) { create(:merge_request, source_project: target, target_project: target) }
-
-        it 'schedules migrations for all its merge requests' do
-          expect(BackgroundMigrationWorker).to receive(:bulk_perform_async).with([['MigrateApproverToApprovalRulesInBatch', ["MergeRequest", [merge_request.id]]]])
-
-          described_class.new.perform(target.class.name, target.id)
-        end
-      end
     end
   end
 end