EmberJS hasMany doesn't fetch relationships

I have a problem with serializing my response which looks like:

{
"data": [
    {
        "id": 930,
        "uniqueId": "0d3a04cb-231c-4998-b4d3-9436a0a3138e",
        "name": "DRINKI",
        "lastEditDate": "2018-02-12T13:30:32",
        "lastEditDateUTC": "2018-02-12T12:30:32",
        "deleted": false,
        "discountable": true,
        "productCategoryPointOfSales": []
    },
    {
        "id": 921,
        "uniqueId": "5fbf423a-4932-47ca-b32f-5d3612dd73ee",
        "name": "BALOTYNKI SOLO",
        "lastEditDate": "2019-02-07T14:20:15",
        "lastEditDateUTC": "2019-02-07T13:20:15",
        "deleted": false,
        "label": "",
        "color": "#a0a5a9",
        "discountable": true,
        "productCategoryPointOfSales": [
            {
                "id": 142,
                "pointOfSaleUniqueId": "98e370f2-9d37-4473-9446-d82e442593fe",
                "directionId": 54,
                "directionUniqueId": "f0c986c0-ef85-4a46-86ea-cd997981fe8a",
                "kitchenUniqueId": "f0c986c0-ef85-4a46-86ea-cd997981fe8a",
                "inactive": false
            }
        ]
    }
],
"total": 0
}

And the error I get:

Encountered a relationship identifier without a type for the hasMany relationship 'productCategoryPointOfSales' on <category:5fbf423a-4932-47ca-b32f-5d3612dd73ee>, expected a json-api identifier with type 'product-category-point-of-sale' but found '{"id":"142","pointOfSaleUniqueId":"98e370f2-9d37-4473-9446-d82e442593fe","directionId":54,"directionUniqueId":"f0c986c0-ef85-4a46-86ea-cd997981fe8a","kitchenUniqueId":"f0c986c0-ef85-4a46-86ea-cd997981fe8a","inactive":false}'. Please check your serializer and make sure it is serializing the relationship payload into a JSON API format.

Models:

export default DS.Model.extend({
productCategoryPointOfSales: DS.hasMany('product-category-point-of-sale'),

uniqueId: DS.attr('string'),
name: DS.attr('string'),
label: DS.attr('string'),
color: DS.attr('string'),
discountable: DS.attr('boolean')
});

export default DS.Model.extend({
category: DS.belongsTo('category'),

pointOfSaleUniqueId: DS.attr('string'),
directionId: DS.attr('string'),
directionUniqueId: DS.attr('string'),
kitchenUniqueId: DS.attr('string'),
inactive: DS.attr('boolean')
});

And my serializer:

export default DS.RESTSerializer.extend(EmbeddedRecordMixin, {
normalizeResponse(store, primaryModelClass, payload, id, requestType) {
payload = {
  category: payload.data,
};

return this._super(store, primaryModelClass, payload, id, requestType);
},
primaryKey: 'uniqueId',

attrs: {
  productCategoryPointOfSales: {embedded: 'always'}
}
});

I'm very new to EmberJS and have no idea how to solve this problem. It looks like root tags for my embedded object are missing. I followed some tutorials and tried with EmbeddedRecordMixin but it didn't help me. Could you please help me figuring this out?