Friendly_id using value from belongs_to association
===

I have the following models:

    class User < ActiveRecord::Base
      extend FriendlyId
      friendly_id :first_name, :use => :slugged
    
      after_create :create_professor
    
      def create_professor
        self.professor = Professor.create 
      end
    
    end 
    
    class Professor < ActiveRecord::Base
      extend FriendlyId
      friendly_id :first_name, :use => :slugged
    
      def first_name
        user.first_name 
      end
    
    end

At the time that `create_professor` is called and therefore the execution passes to the Professor model (`self.professor = Professor.create`), when it gets to try to create the **slug** in this method:

    def first_name
      user.first_name 
    end

I get an `undefined method first_name for nil:NilClass`, so it seems the Professor object doesn't know what is his associated user.

Any tips on how to solve this?
