Rails, before_create - ruby-on-rails

Question

Im rails beginner and I tried to add an carrierwave upload for image urls like shown in the episode #253 from railscast. So far all works good when i insert an normal link, for example
http:/.....g/dog-01.jpg
My problem is that I want to retrieve my images from a javascript plugin that gives a data:image out, how you can see here:
var url = canvas.toDataURL('image/png');
document.getElementById("canvascontent").value = url;
Also you can see that I insert the data:image to an input(#canvascontent).
<%= ff.text_field :remote_name_url, class => "canvascontent" %>
So now my problem, is that when I try to save simply the data:image from the text_field :remote_name_url, this dont works because it is encoded with base64.
The good thing is that Ruby has a Base64 encoder. To encode the data:image and save it in an image file (here png) I wrote a simply code, that I can execute from my console:
require 'base64'
data_url = "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAB....."
png = Base64.decode64(data_url['data:image/png;base64,'.length .. -1])
File.open('testal', 'wb') { |f| f.write(png) }
So to come to the end. My problem is that I don't know how to achieve this with rails!
I know that there is an "code" for the model before_create, i tried something like that, but I didn't worked. I think because i have to safe the image first temporally!
before_create do
require 'base64'
self.remote_name_url = Base64.decode64(remote_name_url['data:image/png;base64,'.length .. -1])
end
class Painting < ActiveRecord::Base
belongs_to :treatment
attr_accessible :name, :image, :remote_name_url
mount_uploader :name, BildUploader
before_create do
require 'base64'
self.remote_name_url = Base64.decode64(remote_name_url['data:image/png;base64,'.length .. -1])
end
end
<
class TreatmentsController < ApplicationController
def create
#patient = Patient.find(params[:patient_id])
#treatment = #patient.treatments.create(params[:treatment])
redirect_to patient_path(#patient)
end
def destroy
#patient = Patient.find(params[:patient_id])
#treatment = #patient.treatments.find(params[:id])
#treatment.destroy
redirect_to patient_path(#patient)
end
end

Answer 1

This article from the Carrierwave wiki seems promising:
https://github.com/carrierwaveuploader/carrierwave/wiki/How-to%3A-Upload-from-a-string-in-Rails-3
There's also the older technique of using Rack::Test::UploadedFile and a StringIO object, but that looks weirder in code that's not intended for the test environment.