gistfile1.txt

var child = new Backbone.Model({name: "child"});
var parent = new Backbone.Model({name: "parent", child: child});
var json = parent.toJson()

This should return:
{
  name: "parent",
  child: {
     name: "child"
  }
}

But it returns:
{
  name: "parent",
  child: Backbone.Model
}