image_uploader.rb

class ImageUploader < CarrierWave::Uploader::Base

  include CarrierWave::RMagick
  storage :file

  version :thumb do
    process :resize_to_fill => [200, 150]
  end

end

post_image.rb

class PostImage < ActiveRecord::Base

  mount_uploader :image, ImageUploader

  validates :image, :presence => true

  validates_integrity_of  :image
  validates_processing_of :image

  attr_accessible :image

end

post_image_spec.rb

require 'spec_helper'

describe PostImage do

  include CarrierWave::Test::Matchers

  context 'without processing' do

    before :all do
      ImageUploader.enable_processing = false # Commenting out this line causes the test to pass!
      @post_image = PostImage.create!(:image => File.open(Rails.root.join('spec', 'support', 'test_image.png')))
    end

    it 'should create an image' do
      @post_image.image.should be_present
    end

  end

  context 'with processing' do

    before :all do
      ImageUploader.enable_processing = true
      @post_image = PostImage.create!(:image => File.open(Rails.root.join('spec', 'support', 'test_image.png')))
    end

    context 'resizing' do

      it 'should create a thumbnail with dimensions 200x150' do
        # Fails with expected "..../thumb_test_image.png" to have an exact size of 200 by 150, but it was 707 by 450.
        @post_image.image.thumb.should have_dimensions(200, 150)
      end

    end

  end

end