json_fieldsets.rb

module ActiveModel
  class Serializer
    class Adapter
      class JsonFieldsets < Adapter
        def initialize(serializer, options = {})
          super

          if fields = options.delete(:fields)
            @fieldset = fields.split(',').map(&:to_sym)
          else
            @fieldset = options[:fieldset]
          end
        end

        def serializable_hash(options = {})
          if serializer.respond_to?(:each)
            @result = serializer.map{|s| self.class.new(s, options.merge(fieldset: @fieldset)).serializable_hash }
          else
            options[:fields] = @fieldset
            @result = serializer.attributes(options)

            serializer.each_association do |name, association, opts|
              if association.respond_to?(:each)
                array_serializer = association
                @result[name] = array_serializer.map { |item| item.attributes(opts) }
              else
                if association
                  @result[name] = association.attributes(options)
                else
                  @result[name] = nil
                end
              end
            end
          end

          if root = options.fetch(:root, serializer.json_key)
            @result = { root => @result }
          end

          @result
        end
      end
    end
  end
end

ActiveModel::Serializer.config.adapter = ActiveModel::Serializer::Adapter::JsonFieldsets

for active model serializers 0.10.0