Question

I’m using backbone for a reasonably complicated form. I have a number of nested models, and have been computing other variables in the parent model like so:

// INSIDE PARENT MODEL

computedValue: function () {
    var value = this.get('childModel').get('childModelProperty');
    return value;
}

This seems to work fine for keeping my UI in sync, but as soon as I call

.save()

on the parent model, I get:

Uncaught TypeError: Object #<Object> has no method 'get' 

It seems that the child model kind of temporarily stops responding.

Am I doing something inherently wrong?

EDIT: The stack trace is:

Uncaught TypeError: Object #<Object> has no method 'get' publish.js:90
Backbone.Model.extend.neutralDivisionComputer publish.js:90
Backbone.Model.extend.setNeutralComputed publish.js:39
Backbone.Events.trigger backbone.js:163
_.extend.change backbone.js:473
_.extend.set backbone.js:314
_.extend.save.options.success backbone.js:385
f.Callbacks.o jquery.min.js:2
f.Callbacks.p.fireWith jquery.min.js:2
w jquery.min.js:4
f.support.ajax.f.ajaxTransport.send.d

EDIT #2 in response to comment below:

There’s something basic I’m still not getting. I replaced a few references to this.get(‘childModel’)[‘childModelProperty’] and now I get things like
‘cannot read property childModelProperty of undefined.

I’m not yet pulling anything from the server, the parent model is just created like

define(['jquery', 'underscore', 'backbone', 'models/childmodel'],    function($, _, Backbone, ChildModel) {
var ParentModel = Backbone.Model.extend({
defaults: {
   childModel : new ChildModel()
    }

Answer 1

defaults are only used when you create the model. After save is called, it will call set which will overwrite childModel with a simple javascript object. As I see it you have a couple options:

1) use Backbone.Relational

2) override set in each parent model to update the existing child model (or create it) something like the following:

children:{
    childModel: ChildModel
}
set: function (key, value, options) {
                var attrs;
                if (_.isObject(key) || key == null) {
                    attrs = key;
                    options = value;
                } else {
                    attrs = {};
                    attrs[key] = value;
                }

                _.each(this.children, function (childType, name) {
                    if (!attrs.hasOwnProperty(name))
                        return;

                    //assume the child is just a model--not a collection
                    var newValue = attrs[name];
                    delete attrs[name];
                    var isModel = this[name] && this[name].set;
                    if (isModel && newValue) {
                        this[name].set(newValue, options);
                    }
                    else if (newValue) {
                        this[name] = new childType(newValue);
                    }
                    else {
                        delete this[name];
                    }

                    this.trigger('change:' + name);

                }, this);

                return Backbone.Model.prototype.set.call(this, attrs, options);             

    }