Add multiple assignees migration and table population

This will be further required for supporting multi-assignees MRs

1 files changed, 36 insertions, 0 deletions

diff --git a/lib/gitlab/background_migration/populate_merge_request_assignees_table.rb b/lib/gitlab/background_migration/populate_merge_request_assignees_table.rb
new file mode 100644
index 00000000000..a4c6540c61b
--- /dev/null
+++ b/lib/gitlab/background_migration/populate_merge_request_assignees_table.rb
@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module BackgroundMigration
+    # This background migration creates records on merge_request_assignees according
+    # to the given merge request IDs range. A _single_ INSERT is issued for the given range.
+    # This is required for supporting multiple assignees on merge requests.
+    class PopulateMergeRequestAssigneesTable
+      def perform(from_id, to_id)
+        select_sql =
+          MergeRequest
+            .where(merge_request_assignees_not_exists_clause)
+            .where(id: from_id..to_id)
+            .where('assignee_id IS NOT NULL')
+            .select(:id, :assignee_id)
+            .to_sql
+
+        execute("INSERT INTO merge_request_assignees (merge_request_id, user_id) #{select_sql}")
+      end
+
+      private
+
+      def merge_request_assignees_not_exists_clause
+        <<~SQL
+            NOT EXISTS (SELECT 1 FROM merge_request_assignees
+                        WHERE merge_request_assignees.merge_request_id = merge_requests.id)
+        SQL
+      end
+
+      def execute(sql)
+        @connection ||= ActiveRecord::Base.connection
+        @connection.execute(sql)
+      end
+    end
+  end
+end