Basically, I have a `Model` type that has configurable `primaryKey`. This is my current implementation that appears broken—I'm guessing the better way to do this would be to build the `primaryKey` property in `init`.

```javascript
var Model = Ember.Object.extend({
  primaryKey: function() {
    return this.get(this.constructor.primaryKey);
  }.property().volatile(),

  isNew: Ember.computed.none('primaryKey')
}).reopenClass({
  primaryKey: 'id'
});

var MyModel = Model.extend().reopenClass({
  primaryKey: 'uuid'
});
```