JSON API

Create sub-resources

The question is, how do we create linked resources with the main one, e.g. if for some business reasons a user is created with an avatar and the avatar is a different resource because it can be associated with other stuff than a user... 

What should be the format of the POST request on /users doing this ? 

json1.json

{
  "users" : [
    "firstname" : "John",
    "lastname" : "Doe",
    "avatar" : [
      "url": "http://foo"
    ]
  ],
  "links": {
	  "friends": ["1", "4", "8"]
  }
}

json2.json

{
  "users" : [
    "firstname" : "John",
    "lastname" : "Doe"
  ],
  "links": {
     "friends" : ["1", "4", "8"],
     "avatar" : [
       "url" : "http://foo"
    ]
  }
}