angoose returns id for populated object

[avandijk]

I retrieve an array of objects including populated reference objects (using find().populate() on server).
Now when I call f.e. a instance method for one of the items in the array all my referenced objects will be resolved to their _id?

f.e. this object (userGroup is a mongoose reference field)

User
name: "Joe",
userGroup: object
{
_id: 57343543534
name: "public"
}
etc..

After I call f.e. an instance method on the server User.test() the User object look like this

User
name: "Joe",
userGroup: 57343543534

etc..

Is this behaviour a bug or is it by design?

thx
Arjan