include?

# File activerecord/lib/active_record/associations/collection_association.rb, line 320
      def include?(record)
        if record.is_a?(reflection.klass)
          if record.new_record?
            include_in_memory?(record)
          else
            load_target if options[:finder_sql]
            loaded? ? target.include?(record) : scoped.exists?(record)
          end
        else
          false
        end
      end