Ember Data with hasMany relation on embedded data - ember.js

Question

I'm using Ember 2.4.1 and trying to build the hierarchical tree with self reference model.
Here is the my Ember model
// app/models/category.js
import DS from 'ember-data';
export default DS.Model.extend({
name: DS.attr('string'),
parent: DS.belongsTo('category', { inverse: 'children' }),
children: DS.hasMany('category', { embedded: 'always', async: false, inverse: 'parent' })
});
And server response in route /categories with embedded data (I use RESTAdapter on Ember side):
{"categories":[
{
"id":4,
"name":"Root element w/o children",
"type":"Category",
"children":[]
},
{
"id":5,
"name":"Root element with a child",
"type":"Category",
"children":[
{
"id":6,
"name":"A child",
"type":"Category",
"children":[]
}
]
}
]}
As you can see Category with id 5 has a child with id 6 and it embedded. But while Ember loading the page it makes an error: Assertion Failed: You looked up the 'children' relationship on a 'category' with id 5 but some of the associated records were not loaded. Either make sure they are all loaded together with the parent record, or specify that the relationship is async (DS.hasMany({ async: true }))
If I include the category with id 6 in JSON-root like {categories: [{id:4, …}, {id:5, …}, {id:6, …}]} error will disappear but I don't know why I need embedding in this case. May be I don't understand how embedding works in Ember.
So I'd like to ask advice how to make properly work Ember Data with embedded response without duplicating it and without async: true.
UPD #TheCompiler answered on my question in comments. Adding serializer with mixin solve my problem:
// app/serializers/category.js
import DS from 'ember-data';
export default DS.RESTSerializer.extend(DS.EmbeddedRecordsMixin, {
attrs: {
children: { embedded: 'always' }
}
});

Answer 1

No need to duplicate the record. The Rest Adapter just doesn't play nice with nested JSON objects. Try formatting the data like this:
{"categories":[
{
"id":4,
"name":"Root element w/o children",
"type":"Category",
"children":[]
},
{
"id":5,
"name":"Root element with a child",
"type":"Category",
"children":[6]
},
{
"id":6,
"name":"A child",
"type":"Category",
"children":[]
}
]}