Ruby on Rails surprise: conditional active_record callbacks

Sometimes you discover hidden features in Rails by trial and error. This week I was refactoring a method into an after_create callback. The method would create an empty user profile when a user was created, thus using an after_create callback would simplify this code. Using the handy create_ method which active_record provides when a belongs_to or has_one relationship is used I've managed to refactor this (quite horrible) piece of code:

class User < ActiveRecord::Base
  has_one :profile

  def create_from_ldap(login)
    #some code left out
    create_personal_belongings
    save
  end

  def create_personal_belongings
    profile = Personal::Profile.new
    profile.id = self.id
    profile.save!
  end
end

into these lines of code:

class User < ActiveRecord::Base
  has_one :profile

  after_create :create_profile

  def create_from_ldap(login)
    #some code left out
    save
  end  
end

Consequence of this change was that a user profile is always created when a user is created. Which is great, because users without a profile should not exist anyway. But it broke some specs, because some specs relied on specifying a certain profile for a user, so we can test some aspects of the user / profile relationship properly.

So what I actually wanted to do, is create a profile after creating a user, but only if the user hasn't got a profile yet.
When using active_record validations I've discovered you can do a conditional validation like this:

class User < ActiveRecord::Base
  validates :email, :presence => true, :email => true, :if => :has_email_enabled?
end

This validates the user's email attribute only if the has_email_enabled? method returns true for the specific user object. (see the active_record_validations documentation for more info) I couldn't find if I can also use conditionals on active_record lifecycle callbacks, so I just tried:

class User < ActiveRecord::Base
  has_one :profile

  after_create :create_profile, :unless => :profile

  def create_from_ldap(login)
    #some code left out
    save
  end  
end

Hey, that works! I believe it's not a documented feature, but it's really nice. I love Rails' adagium "use convention over configuration", because once you know how some stuff works in Rails, you can deduct how the other stuff works. And even better, the chosen conventions usually make sense. To me, at least.