check for model attribute before fetch

I have this line of code in my method in a backbone router.

$.when(system.sideEngine.fetch(), system.lifeSupport.fetch()).done( ...

It works fine if the system has a sideEngineId and a lifeSupportId. However if either id is missing, I get a 404 not found error, and my page won't load.

I tried including an error statement like this:

error: function (model, xhr, options) {
    console.log("something went wrong!");
}

But that's not what I'm looking for. I don't want the absence of ids to be a show stopper. I still want the system to load regardless.

Is there a way to conditionally check for an id before I do a fetch other than creating a complicated if/then tree?

The isNew function should help in this case.

If the model does not yet have an id, it is considered to be new.

var MyModel = Backbone.Model.extend({
  idAttribute: 'whatever',

  fetch: function() {
    $('#result').append('fetch fired!<br/>');
    return null;
  }
});

var noId = new MyModel(),
  hasId = new MyModel({
    whatever: 5
  });

$.when(
  noId.isNew() ? null : noId.fetch(),
  hasId.isNew() ? null : hasId.fetch()
).done(function() {
  $('#result').append('done fired!');
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.8.3/underscore-min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/backbone.js/1.3.3/backbone-min.js"></script>
<div id="result" />

$.when(
    system.sideEngine.isNew() ? null : system.sideEngine.fetch(), 
    system.lifeSupport.isNew() ? null : system.lifeSupport.fetch()
).done(function() {
    ...
});

Note that the null acts like a noop in this case.

If you plan to use this regularly, might be worth considering:

Backbone.Model.prototype.fetchIfId = function() {
    return this.isNew() ? null : this.fetch();
}