how to change store_dir conditionally

[kenan]

I'm trying to upload some images with extension jpg. They are student
pictures where all are named with student numbers.
eg: "11860012.jpg"
For the students who do not have photo, i use a default picture which
is named as "default.jpg". What i want is to upload these pictures to
different directories according to ile names.

eg: while uploading, if the file path is /path/to/file/default.jpg,
store_dir should be "uploads/#{model.class.to_s.underscore}/
#{mounted_as}/defult"

on the other hand if the file path is /path/to/file/11860012.jpg,
store_dir should be "uploads/#{model.class.to_s.underscore}/
#{mounted_as}/#{student_program_code}", where student_program_code is
a method specifies the program code for each student (there are 3
programs and over 5000 students).

Anyway, when i have store_dir method as;

<==================================================================
def store_dir
if original_filename == "default.jpg"
"uploads/#{model.class.to_s.underscore}/#{mounted_as}/default/"
else
"uploads/#{model.class.to_s.underscore}/#{mounted_as}/
#{student_program_code}"
end
end

def student_program_code
student_number = original_filename.split('.').first
year = "20" + original_filename[0,2]
student = Student.find_by_student_number(student_number)
program_code = student.programs.first.code.parameterize
result = [program_code, year].compact.join('_')
result
end
===================================================================>

and with the following rake task;

<===================================================================
desc 'upload student pictures'
task :upload_student_pictures, [:filepath, :program_id]
=> :environment do |t, args|
Student.transaction do
program = Program.find(args.program_id)
program.students.each do |student|
begin
file = File.open("#{args.filepath}/
#{student.student_number}.jpg")
rescue => e
file = File.open("#{args.filepath}/default.jpg")
puts e
ensure
student.picture = file
student.save!
end
end
end
end

=====================================================================>
it assigns the filenames under Student model picture field. But when i
call picture on any student, it results with error


ActionView::Template::Error (You have a nil object when you didn't
expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.split):
4:
5: <table class="list">
6: <tr>
7: <td class="headline"><%=
image_tag(@student.picture.url(:thumb).to_s) unless
@student.picture.url.nil? %></td>
8: <td class="context"></td>
9: </tr>
10: <tr>
app/uploaders/picture_uploader.rb:20:in `store_dir'


when i check original_filename on rails console, i get the
original_filename. But the error says "The error occurred while
evaluating nil.split"..

I could not figured out what is wrong with it..Any help?