# File lib/mongo_mapper/plugins/counter_cache.rb, line 39
        def counter_cache(association_name, options = {})
          options.symbolize_keys!

          field = options[:field] ?
            options[:field] :
            "#{self.collection_name.gsub(/.*\./, '')}_count"

          association = associations[association_name]

          if !association
            raise InvalidCounterCacheError, "You must define an association with name `#{association_name}' on model #{self}"
          end

          # make a define-time check to make sure the counter cache field is defined.
          # note: this can only be done in non-polymorphic classes
          # (since we may not know the class on the other side of the association)
          if !association.polymorphic?
            association_class = association.klass
            key_names = association_class.keys.keys

            if !key_names.include?(field.to_s)
              _raise_when_missing_counter_cache_key(association_class, field)
            end
          end

          after_create do
            if obj = self.send(association_name)
              if !obj.respond_to?(field)
                self.class._raise_when_missing_counter_cache_key(obj.class, field)
              end

              obj.increment(field => 1)
              obj.write_attribute(field, obj.read_attribute(field) + 1)
            end

            true
          end

          after_destroy do
            if obj = self.send(association_name)
              if !obj.respond_to?(field)
                self.class._raise_when_missing_counter_cache_key(obj.class, field)
              end

              obj.decrement(field => 1)
              obj.write_attribute(field, obj.read_attribute(field) - 1)
            end

            true
          end
        end