serializing relationships ember-data

Okay so this is my json response:

item:

{
    "data": {
        "id": 1,
        "username": "rodzzlessa",
        "slug": "rodzzlessa"
     }
}

collection:

{
"data": [
    {
        "id": 34,
        "name": "HDG common nails 50lbs",
        "slug": "hdg-common-nails-50lbs4569",
        "description": "Accusantium ipsam impedit omnis sint dolorum.",
        "image_src": "nail-box-angle.png",
        "categories": {
            "data": [
                {
                     "id": 2,
                     "name": "nails",
                    "image_src": "nails-icon.png"
                }
             ]
         }
     }
  ]
}

I was able to make a serialzier for the main response:

export default DS.RESTSerializer.extend(DS.EmbeddedRecordsMixin, {
    extractArray: function(store, typeClass, payload) {
        payload.products = payload.data;
        delete payload.data;

        return this._super(store, typeClass, payload);
    }
});

Everything works great what I'm having an issue with is the relationships in this case a categories: DS.hasMany('category'). The error I get is:

A (subclass of DS.Model) record was pushed into the store with the value of categories being '{data: [object Object]}', but categories is a hasMany relationship so the value must be an array.

So my questions is which method should I edit to be able to serialize the relationship same way I did to its parent? I've added the DS.EmbeddedRecordsMixin to make my life a lot easier with relationships.