App.Cart.find().then(function (result) { return result.get('firstObject'); });
return the promise to the first object, and then do something with it if it is done. Note that you can use this in the route model method to make it easier to work with promises ember handles.
I used this model. Works great in the latest version of ember.
model: function(params) { return App.Cart.find({slug: params.slug}).then(function (obj) { return obj.get('firstObject'); }); }
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