thoughtbot/factory_girl — comment on commit 90374818a77e20c768c6b8415bdfdeb59304aa2b
File:  line 0
By gamov:

Yes indeed, this doesn't really help because now you have to create two different factories (one for where the association is 'normal' and another one where you state :method => :build).

Still, the foreign key validation problem when you want to save the build model is not solved (validates_presence_of :user_id fails because the associated model user doesn't have an id yet).