I'm having an error trying to add multiple models from the server to a collection. It throws the error:

Uncaught TypeError: Cannot read property 'idAttribute' of undefined

Here is a test example which gives same error:

<script type="text/javascript" src="js/jquery.3.2.1.min.js"></script>
<script type="text/javascript" src="js/underscore.1.8.3.min.js"></script>
<script type="text/javascript" src="js/backbone.1.3.3.min.js"></script>

<script type="text/javascript">

    var Mymodel = Backbone.Model.extend({
        defaults:{
            id: null,
        },
        idAttributes: "id",
        render: function(){
            collection.add([{id:1, name: "name"},{id:2, name: "name"}])
        }
    });

    mymodel = new Mymodel();

    var mycollection = Backbone.Collection.extend({ model: mymodel });

    collection = new mycollection();

    mymodel.render();

</script>

The model property of the collection class should be a model constructor, not an instance.

Backbone.Collection.extend({ model: Mymodel });

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The model class has a idAttribute property, singular, which is already 'id' by default.

---

Putting the id attribute in the defaults isn't necessary and could lead to problems down the road.

---

Other than the previous points, doing rendering within a model is bad practice. Use a view for this.

render: function(){
    collection.add([{ id:1, name: "name" }, { id:2, name: "name" }])
}

I understand this is a test example, but if it wasn't, a model class shouldn't change a global collection instance. The whole point of classes is to scope the responsibilities to the class itself and nothing outside of it.

Using a global like that just goes against the whole point of using Backbone.

Minimal code needed

var MyModel = Backbone.Model.extend({}),
    MyCollection = Backbone.Collection.extend({ model: MyModel }),
    collection = new MyCollection();

collection.add([
    { id: 1, name: "name" },
    { id: 2, name: "name" }
]);