Emberjs : Looking for walkthrough / tutorial on complex models (changed in dropdown)

Seems one thing I cannot get right is when I have a complex model (say "vehicle" that has a "model" property which is a separate model)

What I'm trying to do is have a drop down in the template, listing all models, and have the current model (if any) selected, which I get right.

But when the user selects a different model and save, the vehicle's model property is always null.

For what it matter I can save "simple" or "flat" models just fine.

Seems none of the books I tackled so far address this unless I've been a retard and misunderstand the whole concept. (which is most likely the case!)

ps: my api is asp.net web api running locally. can see the data coming in on put/post. simple members (like string or number) come in just fine. the complex ones (like model in above example) is always null. :-(