Topic: CarrierWave attributes that are not in the database is always nil!

Hi all! I am use Rails 3.2.13 with CarrierWave gem. Why are the attributes that are not in the database is always equal to nil? It is only within the file PostUploader. Ultimately, the data arrives. How to make sure that these attributes are available in the file PostUploader?

class PostUploader < CarrierWave::Uploader::Base
...
  def crop
    model.image_crop # => nil
  end
...
end

model:

mount_uploader :image, PostUploader
attr_accessor  :image_crop, ...
attr_accessible  :image_crop, ...