build

# File activerecord/lib/active_record/associations/join_dependency.rb, line 228
        def build(associations, base_klass)
          associations.map do |name, right|
            reflection = find_reflection base_klass, name
            reflection.check_validity!
            reflection.check_eager_loadable!

            if reflection.polymorphic?
              raise EagerLoadPolymorphicError.new(reflection)
            end

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