# File lib/mongo_mapper/plugins/associations/belongs_to_association.rb, line 18
        def setup(model)
          model.key foreign_key, ObjectId unless model.key?(foreign_key)
          model.key type_key_name, String unless model.key?(type_key_name) if polymorphic?
          super
          add_touch_callbacks if touch?
          add_counter_cache if counter_cache?
        end