Clean up schema of the "merge_requests" table

This adds various foreign keys and indexes to the "merge_requests" table
as outlined in https://gitlab.com/gitlab-org/gitlab-ce/issues/31825.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/31825

1 files changed, 39 insertions, 0 deletions

diff --git a/db/migrate/20171114162227_merge_requests_milestone_id_foreign_key.rb b/db/migrate/20171114162227_merge_requests_milestone_id_foreign_key.rb
new file mode 100644
index 00000000000..c005cf7d173
--- /dev/null
+++ b/db/migrate/20171114162227_merge_requests_milestone_id_foreign_key.rb
@@ -0,0 +1,39 @@
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class MergeRequestsMilestoneIdForeignKey < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  class MergeRequest < ActiveRecord::Base
+    include EachBatch
+
+    self.table_name = 'merge_requests'
+
+    def self.with_orphaned_milestones
+      where('NOT EXISTS (SELECT true FROM milestones WHERE merge_requests.milestone_id = milestones.id)')
+        .where('milestone_id IS NOT NULL')
+    end
+  end
+
+  def up
+    MergeRequest.with_orphaned_milestones.each_batch(of: 100) do |batch|
+      batch.update_all(milestone_id: nil)
+    end
+
+    add_concurrent_foreign_key(
+      :merge_requests,
+      :milestones,
+      column: :milestone_id,
+      on_delete: :nullify
+    )
+  end
+
+  def down
+    remove_foreign_key_without_error(:merge_requests, column: :milestone_id)
+  end
+end