Defaults don't propagate through multiple levels of inheritance

[captncraig]

Consider this code:

var parent = Backbone.Model.extend({defaults: {foo: 7}});
var instance1 = new parent();
console.log(instance1.get('foo')); //7
var child = parent.extend({defaults: {bar: 2}});
var instance2 = new child();
console.log(instance2.get('bar')); //2
console.log(instance2.get('foo')); //undefined

If you extend an existing model and provide new defaults, the original defaults are lost and don't operate as would be intended.
This could possibly be fixed by modifying extend to perform some basic merging with each member of the defaults for both the parent and the child.

[vincentbriglia]

var child = parent.extend({
    defaults: _.extend(parent.prototype.defaults, {
        bar: 2
    })
});

note that parent.extend and _.extend are not the same methods. Also see #276 why this has been declared as the Backbone way.

note that as defaults can be declared as a function you might need something like the following if you're inheriting multiple levels

var child = parent.extend({
    defaults: function () {
        var defaults = parent.prototype.defaults;
        var d = _.isFunction(defaults) ? defaults() : defaults;
        return _.extend(d, {
            bar: 2
        })
    }
});

[simenbrekken]

As a workaround you can assign a function to the defaults attribute and extend that:

Animal = Backbone.Model.extend({
    defaults: {
        eyes: 2,
        legs: 4
    }
});

Dinosaur = Animal.extend({
    defaults: function() {
        return _.extend({
            legs: 2,
            arms: 'too-short'
        }, _.result(Dinosaur.__super__, 'defaults'));
    },
});

var dog = new Animal();
console.log('Dog has:', dog.get('eyes'), 'eyes and', dog.get('legs'), 'legs');

var rex = new Dinosaur();
console.log('Rex has:', rex.get('eyes'), 'eyes and', rex.get('legs'), 'legs');

Will output:

Dog has 2 eyes and 4 legs
Rex has 2 eyes and 2 legs

[braddunbar]

Hi @captncraig. While defaults may not work work like it does in classical inheritance schemes, you can certainly make it behave similarly by declaring it as a function like @sbrekken suggests above. This has been discussed previously in #476 (and is still being discussed there). Would you mind directing further discussion there?