Allow relationships to have an alternative key if relationship key not exist

[danivek]

Some ORM (like js-data) have two keys in object for defining a relation :

• a key with id or ids (unpopulated relation)
• a key with a populated relation

example for an article with a relation with a user :

{
  "id": "1",
  "title": "Nice article",
  "user_id": "2", // unpopulated user
  "user": { // populated user
    "id": "2",
    "name": "John Doe"
  }
}

If we not explicitly populate relation in the ORM, we only have the user_id key in object.

So, relationships configuration could have an alternative key to watch for unpopulated relationships if the populate relationship key does not exist.