I have a before_save method that I call that renames an uploaded image.

before_save :randomize_file_name

def randomize_file_name
  extension = File.extname(screen_file_name).downcase
  key = ActiveSupport::SecureRandom.hex(8)
  self.screen.instance_write(:file_name, "#{key}#{extension}")
end

That method is part of my Item model.

That works great when I create a new item or need to update the image associated with an item...but the problem is that if I need to update an item but NOT the image, the randomize_file_name method still gets run and renames the file in the database (though not the file itself, obviously).

So, I'm thinking I need to figure out a way to only run randomize_file_name if a file is included in the form submission...but I'm not sure how to pull that off.