Working example for https://mail.google.com/mail/u/0/#label/_+Open+Source%2FGrape/14236a566557504a.

exposed_person.rb

require 'spec_helper'
require 'grape'

module DATA
  class User
    attr_accessor :loginname, :email, :person

    def initialize(attrs)
      @loginname = attrs[:loginname]
      @email = attrs[:email]
      @person = attrs[:person]
    end
  end

  class Person
    attr_accessor :name, :avatar, :birthday

    def initialize(attrs)
      @name = attrs[:name]
      @avatar = attrs[:avatar]
      @birthday = attrs[:birthday]
    end
  end
end

module API
  class Person < Grape::Entity
     expose :name, :avatar
  end

  class User < Grape::Entity
     expose :loginname, :email
     expose :person, using: API::Person
  end
end

describe Grape::Entity do
  include Rack::Test::Methods
  subject do
    Class.new(Grape::API) do
      format :json
      get do
        person = DATA::Person.new(name: 'test', avatar: '/uri', birthday: '2013/11/11')
        user = DATA::User.new(loginname: 'test user', email: 'test@test.com', person: person)
        present user, with: API::User
      end
    end
  end
  def app
    subject
  end
  it "works" do
    get '/'
    last_response.status.should == 200
    JSON.parse(last_response.body).should == {"loginname"=>"test user", "email"=>"test@test.com", "person"=>{"name"=>"test", "avatar"=>"/uri"}}
  end
end