primaryKey lives on the serializer, but if we use get(this, 'primaryKey') we get the parent serializer. I think we need to do something like. 
```
var embeddedSerializer = this.store.serializerFor(record.constructor)
get(embeddedSerializer, 'primaryKey')
```
I think it would also be nice to eventually get a nicer api for serializerFor/primaryKey stuff