Question

I’m trying to override Backbone.sync in order to set the “id” attribute every time we fetch a model. This way, I am sure the next model.save() will fire an update and not a create, even if the fetch() didn’t respond with an id.

Backbone._sync = Backbone.sync;
Backbone.sync = function(method, model, options) {
    var params = _.clone(options);
    params.success = function(model) {
        if(method == 'read' && model.isNew()) {
            var id = Math.floor(Math.random()*1000);
            model.set({ "id": id });
        }
        if(options.success) options.success(model);
    };
    Backbone._sync(method, model, params);
}

But the model passed to the success function seems not to be a Backbone model, but just an object. So I can’t use any method (like model.isNew()) on it. Any idea of what I did wrong?

Answer 1

It sounds like you might want to override Model.fetch or Model.parse instead of Backbone.sync. Backbone.sync just takes $.ajax() options, so the success callback is just receiving the JSON response from the server, not an instantiated model.

The default Model.fetch() callback looks like this:

options.success = function(resp, status, xhr) {
    if (!model.set(model.parse(resp, xhr), options)) return false;
    if (success) success(model, resp);
};

Here model refers to the current instance, as you intend. So you might consider overriding Model.fetch to add an id if it’s not there, or, probably better, overriding model.parse(), which is a passthrough by default and is intended as a hook for you to munge the server data. I’m not thrilled with your random number id implementation, as there’s still a chance of collisions, especially if you’re making a lot of models – you might try using model.cid (guaranteed to be unique in the client, though it might have collisions with server-provided IDs) or Underscore’s _.uniqueId():

parse: function(resp, xhr) {
    if (this.isNew() && !resp.id) {
        resp.id = _.uniqueId('model-');
    }
}