Serialize only specific attributes with ActiveModel and Grape API depending on action

Question

In my Grape API the same model can be accessed by different controllers and endpoints. I need to serialize the same model for each of them, but not every attribute applies to all endpoints. I know there is the "filter" method, but this removes an attribute. I would like to list the valid attributes instead. This seems safer. I figured out the following approach. However, is there a built-in way I'm missing?

Using the code below, I would like to return "id, comments, status, user_id" if called by the "/event_signups" endpoint. I would like to return "id, comments, event_id" if called by the "/user_signups" endpoint.

Serializer

module Mobile
  module V4

    class SignupSerializer < ActiveModel::Serializer

      attributes :id,
                 :comments,
                 :status,
                 :event_id,
                 :user_id

      attribute :id,       if: :id?
      attribute :comments, if: :comments?
      attribute :status,   if: :status?
      attribute :event_id, if: :event_id?
      attribute :user_id,  if: :user_id?

      def id?
        !instance_options[:allowed_attributes].index(:id).nil?
      end

      def comments?
        !instance_options[:allowed_attributes].index(:comments).nil?
      end

      def status?
        !instance_options[:allowed_attributes].index(:status).nil?
      end

      def event_id?
        !instance_options[:allowed_attributes].index(:event_id).nil?
      end

      def user_id?
        !instance_options[:allowed_attributes].index(:user_id).nil?
      end

    end

  end
end

Grape API Endpoints

module Mobile
  module V4
    class Signups < Mobile::V4::Root
      include Mobile::V4::Defaults

      resource :signups, desc: 'Operations about the signups' do

        desc 'Returns list of user's signups'
        oauth2 # This endpoint requires authentication

        get '/user_signups', allowed_attributes: [:id, :comments, :event_id] do
          Signup.where(user_id: current_user.id)
        end

        desc 'Returns list of event's signups'
        params do
          requires :event_id, type: Integer, desc: 'Event ID'
        end
        oauth2 # This endpoint requires authentication

        get '/event_signups', allowed_attributes: [:id, :comments, :status, :user_id] do
          Signup.where(event_id: params[:event_id])
        end

      end

    end

  end
end

Answer 1

Can you try selecting only the fields you need? Eg

Signup.where(user_id: current_user.id).select(:id, :comments, :status, :user_id)

Answer 2

I assume that your context does not allow you to use the render function directly. You are facing a problem such as excessively fetching extra fields but you want to fetch some fields defined in fields_array

ActiveModelSerializers::SerializableResource.new(
        resource,
        serializer: SerializerClass,
        fields: fields_array
      ).serializable_hash

Or, if you have a collection of resources, use each_serializer instead of serializer

ActiveModelSerializers::SerializableResource.new(
        collection,
        each_serializer: SerializerClass,
        fields: fields_array
      ).serializable_hash