grape 如何编写有多态关联的 api? 好似直接 expose 是不行的！ picture.rb

class Picture
  include Mongoid::Document
  include Mongoid::Timestamps
  include Mongoid::BaseModel

  mount_uploader :image, ImageUploader

  attr_accessor :uploader_secure_token
  attr_accessible :image

  belongs_to :imageable, :polymorphic => true
end

product.rb

class Product
  has_many :pictures, :as => :imageable, :dependent => :destroy
end

entity.rb

class Product < Grape::Entity
   expose :id, :name, :cover
end

class DetailProduct < Grape::Entity
   expose :id, :name, :howmany, :price, :exprice, :dprice, :mprice, :inprice, :size, :use, :feature, :inventory,
                    :sales, :cover, :selling, :barcode, :special_offer
   expose :pictures, :using => APIEntities::Picture
end

class Picture < Grape::Entity
  expose :image
end

api.rb

resource :products do
      get do
        @products = Product.limit(20)
        present @products, :with => APIEntities::DetailProduct
      end

      get ":id" do
        @product = Product.includes(:pictures).find(params[:id])
        present @product, :with => APIEntities::DetailProduct
      end
end