# File lib/mongoid/relations/conversions.rb, line 21
      def flag(object, metadata)
        inverse = metadata.inverse_klass
        if inverse.using_object_ids? || object.is_a?(BSON::ObjectId)
          object
        else
          object.tap do |obj|
            if obj.is_a?(String)
              obj.unconvertable_to_bson = true
            end
          end
        end
      end