GrapeでAPIを構築時のメモ ref: http://qiita.com/taizawa/items/a5cce420298e92b046aa

file0.txt

class Post < ActiveRecord::Base
  has_many :post_relationships
  has_many :categories, through: :post_relationships
  validates :name, presence: true
  accepts_nested_attributes_for :categories
end

class PostRelationship < ActiveRecord::Base
  belongs_to :post
  belongs_to :category
end

class Category < ActiveRecord::Base
  has_many :post_relationships
  has_many :posts, through: :post_relationships
end

file1.txt

# api.py
module API
  class Base < Grape::API
    prefix 'api'
    version 'v1', using: :path
    format :json

    mount Post_API
  end
end

# post_api.py
class Post_API < Grape::API
  resource :posts do

    # http://localhost:3000/api/v1/posts
    get do
      # Post.all() ←これだとカテゴリーの中身が返らない。
      # これでカテゴリー含めて返る
      Post.all()as_json(include: :categories)
    end

    # http://localhost:3000/api/v1/posts/1
    get ':id' do
      Post.find(params[:id])
    end
  end
end