[Rails] error: uninitialized constant

[Sebastian Friedrich]

total rails nuby here. I have a table "thumbnails", hence the model  
Thumbnail. I'm extending the Thumbnail class with a few constants  
needed for thumbnail cropping:

class Thumbnail < ActiveRecord::Base

SOURCE_FILE_PATH = "path/to/source"
TARGET_FILE_PATH = "path/to/target"
TARGET_WIDTH = 100
TARGET_HEIGHT = 100

end

Now, in my controller i'm trying to access these:

source_path = Thumbnail::SOURCE_FILE_PATH

however, i get an error:

uninitialized constant SOURCE_FILE_PATH
/usr/local/lib/ruby/gems/1.8/gems/activesupport-1.2.5/lib/ 
active_support/dependencies.rb:200:in `const_missing'
This error occured while loading the following files:  
source_file_path.rb

so, it seems it interprets the scope operator as a request for class  
SOURCE_FILE_PATH in module Thumbnail. Doing this straight in Ruby  
works fine, however. I've also seen a constant being called that way  
from the View in the Agile book. anybody could enlighten me on what  
is different here?

is this a bad way of dealing with "hardwired" data in general? should  
this be in its own model even though its very much related to the  
"thumbnail" logic? I'm still having a somewhat hard time to wrap by  
head around the various concepts of separation, so please forgive me  
for sounding stupid. any good suggestions on how to incorporate that  
the Rails-way.

thanks.
sebastian