Loading derived objects from a parent model

I have a base model object that that has one of several possible derived models which I need to load so that I can access the specific details pertaining to the child object.

I’m following the approach of using a computed property within the parent model that looks up the type and then uses that to perform a lookup on derived model. See Edit fiddle - JSFiddle - Code Playground for an example.

The problem is that within the detail hook of the parent, the model attributes that I need to use are always undefined.

Any thoughts on why is happening, or if there is another way to achieve a parent to child lookup?