CarrierWave Error

I get an error 开发者_StackOverflow社区that looks like this:

   NoMethodError (undefined method `image_will_change!' for #<Bar:0x24ddda0>):
  app/controllers/bars_controller.rb:43:in `new'
  app/controllers/bars_controller.rb:43:in `create'

After I click submit for a registration. I am using Carrierwave and have followed the steps to using it, but still have an error.

CarrierWave doesn't support the attribute_will_change! call, at least I ran into this error a few times. Follow the steps from How to: Detect a new file in a mounted uploader at CarrierWave's wiki which provide similar functionality.