Question

var sgt1 = Backbone.Collection.extend({
        model: sgt2
    });

var a = new sgt1();
a.add({attr1: 'asd'});

Ok, up to now it is works but:

a.get(0) returns undefined

and

a.at(0) returns an obj

Why does get return undefined?

Answer 1

a.get(0) fetches by ID. It is not the same as at which fetches by index position. Since your model does not yet have an ID, get would not work.