Cannot alter model attribute directly when carrierwave uploader is mounted - ruby-on-rails

Question

I am using activeadmin, and the following code is supposed to assign a photo to a Card if found in a directory.
The Card model won't accept the new photo, it never assigns the new value, save returns true.
UPDATE: The problem is that the attribute has a carrierwave uploader mounted on it. Commenting out the mount made it work, but it really isn't an acceptable solution since this is an action that will have to be made repeatedly... Is there a way to unmount the uploader or set the attribute value directly with the uploader mounted? I can't seem to find anything on that on the web...
UPDATE: Even ActiveRecord::Base.connection.execute("update cards set photo='#{f.to_s}' where id=#{card.id};") fails when carrierwave is mounted... It's taken over for good I guess
UPDATE: It turns out my question is a duplicate of
Manually updating attributes mounted by Carrierwave Uploader
from the log:
Card Load (0.5ms) SELECT `cards`.* FROM `cards` WHERE `cards`.`gatherer_id` = 244675 LIMIT 1
(0.2ms) BEGIN
Card Load (0.4ms) SELECT `cards`.* FROM `cards` WHERE `cards`.`id` = 1377 LIMIT 1
SQL (0.5ms) UPDATE `cards` SET `photo` = NULL, `updated_at` = '2013-11-22 11:20:44' WHERE `cards`.`id` = 1377
(4.1ms) COMMIT
After carrierwave is not mounted and a value for the attribute is updated just fine and afterwards, when carrierwave is mounted again it returns the file location according to the uploader store_dir normally. The only problem is I cannot set the value when the uploader is mounted. As a last resort I will do it with plain old sql, I just don't like the idea much.
activeadmin action:
ActiveAdmin.register_page 'Import FTP photos' do
menu label: 'Import FTP photos', parent: 'Cards'
#action_item do
# link_to 'View Site', '/'
#end
content do
require 'fileutils'
para 'Importing photos...'
from_dir = 'var/uploads/card_photos'
uploads_dir = 'public/uploads/card/photo/'
Dir.foreach(from_dir) do |f|
next if f == '.' or f == '..'
from_file = from_dir + '/' + f.to_s
id = f.gsub(/\.\w+$/, '')
card = Card.find_by_gatherer_id(id)
if card
to_dir = uploads_dir+card.id.to_s
to_file = to_dir + '/' + f.to_s
Dir.mkdir(to_dir) if Dir.new(to_dir).nil?
FileUtils.remove(to_file) if FileTest.exists?(to_file)
FileUtils.mv(from_file, to_file)
card.photo = f.to_s
card.save
end
end
end
end
card model
class Card < ActiveRecord::Base
mount_uploader :photo, PhotoUploader
belongs_to :card_set
belongs_to :card_artist
def to_s
caption
end
private
def card_params
params.permit!
end
end