# File lib/mongo_mapper/plugins/associations/belongs_to_polymorphic_proxy.rb, line 6
        def replace(doc)
          if doc
            doc.save unless doc.persisted?
            id, type = doc.id, doc.class.name
          end

          proxy_owner[association.foreign_key] = id
          proxy_owner[association.type_key_name] = type
          reset
          unless doc.nil?
            loaded
            @target = doc
          end
          @target
        end