Save reference of parent object in child object and retrieve it

```
class Article
  include Locomotive::Entity
  attributes :title, :content, :author, :comments
end

class Comment
  include Locomotive::Entity
  attributes :title, :content, :article_id
end

require_relative 'lib/locomotive/adapters/memory_adapter'
adapter = Locomotive::Adapters::MemoryAdapter

mapper = Locomotive::Mapper.new(adapter) do
	collection :articles do
	  entity Locomotive::Example::Article
	  repository Locomotive::Example::ArticlesRepository

	  attribute :title, localized: true
	  attribute :content,  klass: String

	  attribute :comments, association: :comments
	end

	collection :comments do
	  entity Locomotive::Example::Comment
	  repository Locomotive::Example::CommentsRepository

	  attribute :title
	  attribute :content
	  attribute :article_id
	end
end

Comment.new( { title: 'awesome comment', content: 'Lorem ipsum dolor sit amet, ...' })
Article.new(title: { en:'My title' }, content: 'The article content', comments: [])

articles_repository.create article
article.id.should_not be_nil

article.comments << comment
articles_repository.update article

comment.article_id.should eq article.id

article_double = articles_repository.find(article.id)
comment = article_double.comments.first

comment.article_id.should eq article.id
```