Rails: Passing variable from form to model

Rails: Passing variable from form to model

I have a form, where users can upload a picture and can choose a watermark
from a list. The value in the field is then the URL of the watermark (for
simplification in development). I need to pass the chosen watermark_url to
the model, so that rmagick can add the watermark. I use paperclip for the
uploads.
In my model I have the following:
class Image < ActiveRecord::Base
...
attr_accessor :watermark_url
has_attached_file :picture, :processors => [:watermark], styles: {
thumb: '100x100>',
medium:{
geometry: '300x300>',
watermark_path: Rails.root.join(watermark_url),
position: "Center"}
}
end
watermark_url is a field in the form and in the database (which would not
be necessary). Then I get the following error: undefined local variable or
method `watermark_url'.
Am i missing something here?
Thank you in advance for helping!