build

# File activerecord/lib/active_record/associations/join_dependency.rb, line 214
      def build(associations, base_klass)
        associations.map do |name, right|
          reflection = find_reflection base_klass, name
          reflection.check_validity!

          if reflection.options[:polymorphic]
            raise EagerLoadPolymorphicError.new(reflection)
          end

          JoinAssociation.new reflection, build(right, reflection.klass)
        end
      end