title: after_add callback for has_one
body: <p>This seems to be an inconsistency between has_many and has_one.</p>
  
  <p>The has_many association allows you to specify an after_add callback that is called after an object has been added to the collection. </p>
  
  <pre><code>class Person
    has_many :parents, :after_add =&gt; { puts "Added new parent" } # allowed
    has_one :car, :after_add =&gt; { puts "Added car" } # not allowed
  end
  
  class Car
    after_create :assign_name
    def assign_name
      self.name = "Herbie"
    end
  end
  </code></pre>
  
  <p>Unfortunately, there isn't an after_add callback for the has_one association. How do you achieve the same thing for a has_one?</p>