Downloading with Carrierwave works in the rails console, but not in the specification

I have the following model:

class Face < ActiveRecord::Base
  attr_accessible :face_index, :design, :background
  belongs_to :template
  mount_uploader :background, BackgroundUploader
end

BackgroundUploader:

class BackgroundUploader < CarrierWave::Uploader::Base
  def store_dir
    "uploads/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"
  end
end

When I launch the rails console, I can create a Face and save the background for it:

f = Face.create(:face_index => 0)
f.background = File.open("/path/to/image.jpg")
f.save!

Everything works, but when I try to move it to rspec, I get a failure:

Failures:

  1) Face A new face 
     Failure/Error: @face.background = File.open(image_path)
     NoMethodError:
       undefined method `background_will_change!' for #<Face:0x007ff63d9f7410>

Specification:

describe Face do

  before(:each) do
    image_path = Rails.root.join('spec/support/images', '02.jpg').to_s
    @face = FactoryGirl.create(:face)
    @face.background = File.open(image_path)
    @face.save!
  end

  describe "A new face" do
    it { should belong_to(:template) }
  end

end

factory:

FactoryGirl.define do    
  factory :face do
    face_index 0
  end
end

I saw this error before the loader column was missing in db, but if my migrations are correct for dev, they should be correct for testing, not? Do I need to require something in the specification to make it work?

thank!