Save belongsTo change in model not part of Payload - ember.js

Question

I'm trying to save a change to a parent model on a child model in Ember to my server, but for some reason the REST Payload doesn't contain the belongsTo Relationship
My 2 models are defined like this:
parent.js
import DS from 'ember-data';
export default DS.Model.extend({
parentName: DS.attr('string')
});
child.js
import DS from 'ember-data';
export default DS.Model.extend({
childName: DS.attr('string'),
parent: DS.belongsTo('parent')
});
when I change the belongsTo to a different parent for a childModel by editing and saving an existing record. Somehow my payload doesnt include the parent model.
For example.
I have a child like this:
{
id: 1,
parent: 1,
childName: "Child 1"
}
and in code I do this:
childModel.set('parent', parentModel); // this is a different parentModel, one with id: 2
I would expect the payload to look something like this:
HTTP-PUT: http://server/child/1
{
id: 1,
parent: 2,
childName: "Child 1"
}
however, in reality, the payload is this:
HTTP-PUT: http://server/child/1
{
id: 1,
childName: "Child 1"
}
What goes wrong here? Why is the parent relationship missing from the payload?
Some extra information:
Ember v2.0.1
Ember-data v2.0.0
Relationships must be async: true (which is the default)
I'm using a standard DS.JSONAPISerializer in combination with the DS.RestAdapter

Answer 1

Turns out during the serialization, I only extracted the attributes hash from my JSON Api - JSON, Obviously I need to look for relationships in the relationships hash.