Take the following for example:

```
class Foo < AR::Base
  has_many :bars, :as => :barable, :dependent=> :destroy
  accepts_nested_attributes_for :bars, :allow_destroy => true
end

class Bar < AR::Base
   belongs_to :barable, :polymorphic => true  
end

class Baz < Bar

  before_save do 
    raise "Hi"
  end

end

```

In the form for 'Foo' - I have `fields_for :bars_attributes` where a hidden field sets `type` to 'Baz'. The 'Baz' is succesfully created but the callback never fires. (It does, however, fire when manually creating a 'Baz' in the console.)

Any advice appreciated!