When to useval or def in Scala traits?

Question

I was going through the effective scala slides and it mentions on slide 10 to never use val in a trait for abstract members and use def

Answer 1

Always using def seems a bit awkward since something like this won't work:

trait Entity { def id:Int}

object Table { 
  def create(e:Entity) = {e.id = 1 }  
}

You will get the following error:

error: value id_= is not a member of Entity