Get relationship id from the resource

[gottfrois]

We should avoid getting relationship id from the model directly to allow clients to customize how relationship ids are fetched. The resource exposes the relationship id through a method which delegates to the model. This way we allow clients to override the relationship id in the resource to fit their needs.

Today, by default, the resource object already delegate the relationship id method to the model:

class AuthorResource
end

class MessageResource
  has_one :author
end

message.author_id
=> 1
message._model.author_id
=> 1

This way, a client can override the relationship id method:

class MessageResource
  has_one :author

  def author_id
    @model.author.some_custom_id
  end
end

[knapo]

+1

[lgebhardt]

@gottfrois Thanks!