Add #started_at/#finished_at to background data migrations

lib/online_migrations/background_migrations/migration.rb

@@ -180,6 +180,20 @@ def retry_failed_jobs
         end
       end
 
+      # Returns the time this migration started running.
+      def started_at
+        # To be precise, we should get the minimum of `started_at` amongst the children jobs
+        # (for simple migrations) and amongst the children migrations (for composite migrations).
+        # But we do not have an appropriate index on the jobs table and using this will lead to
+        # N+1 queries if used inside some dashboard, for example.
+        created_at
+      end
+
+      # Returns the time this migration finished running.
+      def finished_at
+        updated_at if completed?
+      end
+
       # @private
       def on_shard(&block)
         abstract_class = Utils.find_connection_class(migration_model)

test/background_migrations/migration_test.rb

@@ -244,6 +244,19 @@ def test_retry_failed_jobs
       assert m.enqueued?
     end
 
+    def test_started_at
+      m = create_migration
+      assert_equal m.created_at, m.started_at
+    end
+
+    def test_finished_at
+      m = create_migration
+      assert_nil m.finished_at
+      2.times { run_migration_job(m) }
+      assert m.succeeded?
+      assert_equal m.updated_at, m.finished_at
+    end
+
     def test_next_batch_range
       user1, user2, user3 = 3.times.map { User.create! }
       m = create_migration(batch_size: 2, sub_batch_size: 1)