Save nested models using Ember Data

I have two models in Ember:

Collection

export default DS.Model.extend({
    name: DS.attr(),
    description: DS.attr(),
    items: DS.hasMany('collection-item')

});

Collection Item

export default DS.Model.extend({
    date: DS.attr(),
    volume: DS.attr(),
    sequenceNumber: DS.attr()
});

I want to save the collection items inside the 'items' attribute of the collection, like MongoDB:

[{
    "name": "First Collection",
    "description": "This is my first collection",
    "items": [
        {
            "date": "2017-07-26",
            "volume": "1",
            "sequenceNumber": "1"
        },
        {
            "date": "2017-07-27",
            "volume": "1",
            "sequenceNumber": "2"
        }
    ]
},
{
    "name": "Second Collection",
    "description": "This is my second collection",
    "items": [
        {
            "date": "2017-07-26",
            "volume": "1",
            "sequenceNumber": "1"
        },
        {
            "date": "2017-07-27",
            "volume": "1",
            "sequenceNumber": "2"
        }
    ]
}]

I have read something about serializers, but I don't get the point ;) Can someone give me a hint?

BTW, I'm using Firebase (emberfire) for now, but I'm going to build my own API in future.