1 files changed, 86 insertions, 0 deletions
diff --git a/app/models/concerns/deprecated_assignee.rb b/app/models/concerns/deprecated_assignee.rb
new file mode 100644
index 00000000000..7f12ce39c96
--- /dev/null
+++ b/app/models/concerns/deprecated_assignee.rb
@@ -0,0 +1,86 @@
+# frozen_string_literal: true
+
+# This module handles backward compatibility for import/export of Merge Requests after
+# multiple assignees feature was introduced. Also, it handles the scenarios where
+# the #26496 background migration hasn't finished yet.
+# Ideally, most of this code should be removed at #59457.
+module DeprecatedAssignee
+  extend ActiveSupport::Concern
+
+  def assignee_ids=(ids)
+    nullify_deprecated_assignee
+    super
+  end
+
+  def assignees=(users)
+    nullify_deprecated_assignee
+    super
+  end
+
+  def assignee_id=(id)
+    self.assignee_ids = Array(id)
+  end
+
+  def assignee=(user)
+    self.assignees = Array(user)
+  end
+
+  def assignee
+    assignees.first
+  end
+
+  def assignee_id
+    assignee_ids.first
+  end
+
+  def assignee_ids
+    if Gitlab::Database.read_only? && pending_assignees_population?
+      return Array(deprecated_assignee_id)
+    end
+
+    update_assignees_relation
+    super
+  end
+
+  def assignees
+    if Gitlab::Database.read_only? && pending_assignees_population?
+      return User.where(id: deprecated_assignee_id)
+    end
+
+    update_assignees_relation
+    super
+  end
+
+  private
+
+  # This will make the background migration process quicker (#26496) as it'll have less
+  # assignee_id rows to look through.
+  def nullify_deprecated_assignee
+    return unless persisted? && Gitlab::Database.read_only?
+
+    update_column(:assignee_id, nil)
+  end
+
+  # This code should be removed in the clean-up phase of the
+  # background migration (#59457).
+  def pending_assignees_population?
+    persisted? && deprecated_assignee_id && merge_request_assignees.empty?
+  end
+
+  # If there's an assignee_id and no relation, it means the background
+  # migration at #26496 didn't reach this merge request yet.
+  # This code should be removed in the clean-up phase of the
+  # background migration (#59457).
+  def update_assignees_relation
+    if pending_assignees_population?
+      transaction do
+        merge_request_assignees.create!(user_id: deprecated_assignee_id, merge_request_id: id)
+        update_column(:assignee_id, nil)
+      end
+    end
+  end
+
+  def deprecated_assignee_id
+    read_attribute(:assignee_id)
+  end
+end