carrierwave trouble

test.rb

# this doesn't throw error, but now file gets uploaded, nothing stored in the blog_image string field (that's my mounted uploader where carrierwave should store the image identifier in)

      img_path = "#{RAILS_ROOT}/tmp/sets/"
      img_name = "set_#{self.id}_#{Time.now.to_i}.png"
      
      # save composite image to PNG
      template.write(img_path + img_name)  # this works, it generates the desired PNG
      #save and upload to s3
      
      tmp_image = File.open(img_path + img_name)
      
      #self.blog_image = File.open(img_name)
      
      self.blog_image.store!(self.blog_image.store_path(img_name))
      
      self.write_blog_image_identifier
      self.save!


# this is what i tried before and now again, it says "No such file or directory - set_49_1310723500.png"
# also this file is generated and exists in APP_ROOT/tmp/sets/ ?? maybe i am using File.open wrong?

     img_path = "#{RAILS_ROOT}/tmp/sets/"
      img_name = "set_#{self.id}_#{Time.now.to_i}.png"
      
      # save composite image to PNG
      template.write(img_path + img_name)  # this works, it generates the desired PNG
      #save and upload to s3
      
      tmp_image = File.open(img_path + img_name)
      
      self.blog_image = File.open(img_name)
      
      self.save!