REST API sample by Grape & Grape::Entity. return nested items.

entities.rb

module TEST::Entities
  class Post < Grape::Entity
    expose :id
    expose :title
    expose :description
    expose(:image_url) do |post, options|
      if post.image.url.present?    # else is return null
          "http://#{options[:env]['HTTP_HOST']}"+"#{post.image.url}"
      end
    end
    expose(:like_count) do |post, options|
        post.likes.size
    end
    expose(:comment_count) do |post, options|
        post.comments.size
    end
    expose(:tags) {|post| post.tag_list || [] }
    expose :user, :using => TEST::Entities::UserBasic
  end

  class UserBasic < Grape::Entity
    expose :id
    expose :username
    expose :description
  end

  class UserDetail < UserBasic
    expose :email
  end
end

posts.rb

class TEST::Posts < Grape::API

  resource "posts" do

    desc "returns all posts"
    params do
      optional :page, type: Integer, desc: "page", default:1
      optional :per_page, type: Integer, desc: "per page" ,default:30
    end
    get do
      posts = Post.page(params[:page]).per(params[:per_page])
      present :total_count, posts.size
      present :total_page_count, posts.total_pages
      present :page, posts.current_page
      present :posts, posts, with: Proj::Entities::Post
              
    end                                                        

  end
end

return.json

{
  "total_count": 50,
  "total_page_count": 2,
  "page": 1,
  "per_page": 30,
  "posts": [
    {
      "id": 10,
      "title": null,
      "description": null,
      "image_url": "http://test.dev/uploads/post/image/10/hoge.png",
      "like_count": 0,
      "comment_count": 0,
      "tags": [
        "aaaa",
        "bbbb"
      ],  
      "user": {
        "id": 1,
        "username": "user1",
        "description": "description1",
      }
    },
    .........
  ]
}