bad_test.rb

require 'rubygems'
require 'mongo_mapper'
require 'joint'
require 'shoulda'

MongoMapper.database = 'joint_exif_test'

class Photo  
  include MongoMapper::Document   
  plugin Joint  
  attachment :image
end

class TestJointExif < Test::Unit::TestCase  
  context 'given an image' do     
    setup do 
      MongoMapper.database.collections.each(&:remove)      
      file = File.open('/Users/fabrizioregini/Desktop/bikes.jpg', 'r')
      @photo = Photo.create( :image => file )
    end
    
    subject { @photo }
    
    should 'remove attachment' do   
      subject.image = nil
      subject.save 
      assert subject.image? == false # This line fails
    end
  end
end