Fetch last migration job by max_value

When we split failed jobs, the job with the highest ID will no longer be
the one with the highest max_value

lib/gitlab/database/background_migration/batched_migration.rb

@@ -10,7 +10,7 @@ module Gitlab
         self.table_name = :batched_background_migrations
 
         has_many :batched_jobs, foreign_key: :batched_background_migration_id
-        has_one :last_job, -> { order(id: :desc) },
+        has_one :last_job, -> { order(max_value: :desc) },
           class_name: 'Gitlab::Database::BackgroundMigration::BatchedJob',
           foreign_key: :batched_background_migration_id
 

spec/lib/gitlab/database/background_migration/batched_migration_spec.rb

@@ -10,11 +10,11 @@ RSpec.describe Gitlab::Database::BackgroundMigration::BatchedMigration, type: :m
 
     describe '#last_job' do
       let!(:batched_migration) { create(:batched_background_migration) }
-      let!(:batched_job1) { create(:batched_background_migration_job, batched_migration: batched_migration) }
-      let!(:batched_job2) { create(:batched_background_migration_job, batched_migration: batched_migration) }
+      let!(:batched_job1) { create(:batched_background_migration_job, batched_migration: batched_migration, max_value: 1000) }
+      let!(:batched_job2) { create(:batched_background_migration_job, batched_migration: batched_migration, max_value: 500) }
 
-      it 'returns the most recent (in order of id) batched job' do
-        expect(batched_migration.last_job).to eq(batched_job2)
+      it 'returns the batched job with highest max_value' do
+        expect(batched_migration.last_job).to eq(batched_job1)
       end
     end
   end