Custom upload_to callback which using given names of fields to obtain access to storage and input data to create a slug

upload_utils.py

import os
from django.template.defaultfilters import slugify
try: # make it python2.4 compatible!
    from hashlib import md5
except:
    import md5


def picture_filename(path, field_name, field_to_slug = None):
    def upload_to(instance, filename):
        field = instance._meta.get_field(field_name)

        if field_to_slug:
            new_fname = slugify(getattr(instance, field_to_slug, ''))
            
            if new_fname != '':
                filename = u''.join((new_fname, os.path.splitext(filename)[1]))
        
        m = md5()
        m.update(filename)
        
        filename = os.path.join(path, m.hexdigest()[:2], field.get_filename(filename))

        if len(filename) > field.max_length:
            basename, ext = os.path.splitext(filename)
            
            # +4 to ensure, that _nnn added by storage backend to filename won't overcome field limit
            basename = basename[:field.max_length - len(ext) + 4] 
            filename = u''.join((basename, ext))

        return filename
        
    return upload_to