simple html value for nested form - ruby-on-rails-3

Question

I'm using rails, and backbone with handlebars, and I have a nested form.
I am trying to create a user, and the user can supply a url.
So I have a user model and a url model
I thought I could do something like this
<form id="new_user">
<label name="username" >username</label>
<input type="text" name="username" />
<label name="url" >url</label>
<input type="text" class="nested urls_attributes" name="url" />
</form>
----------------------update -----------------------------------------
as this is something that will be seldomly used, I've updated my backbone view to create the correct json when I submit the form, so now on submit form, I create the first json, then add the nested object.
var form_json = MyApp.Helpers.Serialize_Form($('form#user_form inputs').not('.nested'));
form_json['urls_attributes']=MyApp.Helpers.Serialze_Form($('form#user_form inputs.nested'));
this creates the json in the format of
{username: "test", urls_attributes: {url:"http://test"}}
So when I pass that to my controller, I expected that the url would be created when the user is created.
Unfortunately, that doesn't seem to be happening, and I have no errors. The user gets created, but not the nested model of urls.
Any idea why this is happening?
-----------how I make the json----------------------
I convert my form into a json object with
serialize_objects: function(inputs){
//this takes form inputs and creates a json string of them
var o = {};
$.map(inputs, function(t) {
if (o[t.name] !== undefined) {
if (!o[t.name].push) {
o[t.name] = [o[t.name]];
}
o[t.name].push(t.value || '');
} else {
o[t.name] = t.value || '';
}
});
return o;
}
I can't use jquery serializeArray because I'm using jqMobi which doesn't implement serializeArray. But that shouldn't matter for this question.