Hanami with roar - Nested JSON API implement

book.rb

class Book < Hanami::Entity

  def author
    AuthorRepository.new.find_with_book(self)
  end

  def comments
    CommentRepository.new.find_with_book(self)
  end
end

book_representer.rb

require 'roar/decorator'
require 'roar/json'
require_relative 'comment_representer' # required

module Api::Representers
  class BookRepresenter < Roar::Decorator
    include Roar::JSON

    property :id
    property :title
    property :author, extend: Author , class: Author
    property :description
    collection :comments, extend: CommentRepresenter , class: Comment
  end
end

comment_repository.rb

class CommentRepository < Hanami::Repository
  def find_with_book(book)
    comments.where(book_id: book.id)
  end
end

index.rb

module Api::Controllers::Books
  class Index
    include Api::Action

    expose :books

    def call(params)
      repo = BookRepository.new
      books = repo.all
      hashes = books.map do |book|
        Api::Representers::Book.new(book).to_hash
      end

      self.status = 200
      self.body = { books: hashes }.to_json
    end
  end
end

migration.rb

Hanami::Model.migration do
  change do
    create_table :books do
      primary_key :id
      foreign_key :author_id, :authors
      column  :title, String
      column  :description, String
      column  :created_at, DateTime
      column  :updated_at,  DateTime
    end
    create_table :comments do
      primary_key :id
      foreign_key :book_id, :books
      column  :body, String
      column  :created_at, DateTime
      column  :updated_at,  DateTime
    end
    create_table :authors do
      primary_key :id
      column  :name, String
      column  :created_at, DateTime
      column  :updated_at,  DateTime
    end
  end
end