1 files changed, 49 insertions, 0 deletions
diff --git a/db/post_migrate/20200831065705_ensure_target_project_id_is_filled.rb b/db/post_migrate/20200831065705_ensure_target_project_id_is_filled.rb
new file mode 100644
index 00000000000..9b267933b04
--- /dev/null
+++ b/db/post_migrate/20200831065705_ensure_target_project_id_is_filled.rb
@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+class EnsureTargetProjectIdIsFilled < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  BACKGROUND_MIGRATION_CLASS = 'CopyMergeRequestTargetProjectToMergeRequestMetrics'
+  BATCH_SIZE = 1_000
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  class MergeRequest < ActiveRecord::Base
+    self.table_name = 'merge_requests'
+  end
+
+  class MergeRequestMetrics < ActiveRecord::Base
+    include EachBatch
+
+    belongs_to :merge_request
+
+    self.table_name = 'merge_request_metrics'
+  end
+
+  def up
+    Gitlab::BackgroundMigration.steal(BACKGROUND_MIGRATION_CLASS)
+
+    # Do a manual update in case we lost BG jobs. The expected record count should be 0 or very low.
+    MergeRequestMetrics.where(target_project_id: nil).each_batch do |scope|
+      query_for_cte = scope.joins(:merge_request).select(
+        MergeRequestMetrics.arel_table[:id].as('id'),
+        MergeRequest.arel_table[:target_project_id].as('target_project_id')
+      )
+
+      MergeRequestMetrics.connection.execute <<-SQL
+        WITH target_project_id_and_metrics_id as (
+          #{query_for_cte.to_sql}
+        )
+        UPDATE #{MergeRequestMetrics.connection.quote_table_name(MergeRequestMetrics.table_name)}
+        SET target_project_id = target_project_id_and_metrics_id.target_project_id
+        FROM target_project_id_and_metrics_id
+        WHERE merge_request_metrics.id = target_project_id_and_metrics_id.id
+      SQL
+    end
+  end
+
+  def down
+    # no-op
+  end
+end