How to test after_initialize callback of a rails model?

Question

I am using FactoryGirl and Rspec for testing. The model sets a foreign key after init if it is nil. Therefore it uses data of another association. But how can I test it? Normally I would use a factory for creation of this object and use a stub_chain for "self.user.main_address.country_id". But with the creation of this object, after initialize will be invoked. I have no chance to stub it.

after_initialize do
  if self.country_id.nil?
    self.country_id = self.user.main_address.country_id || Country.first.id
  end
end

Any idea?