passing a callback as upload_to to FileField

Question

I have an abstract model class UploadItem for handling uploaded files. I want each subclass to be able to define the upload_to path. For this, i pass a callback to the constructor of FileField.

This is an example:

class UploadItem(models.Model):
    file = models.FileField(upload_to=UploadItem.get_directory) 


    class Meta:
        abstract = True
# I want videos to be storred in 'videos/' directory
class Video(UploadItem):
    def get_directory(self, instance, filename):
        return 'videos/'

But this doesn't work, i am getting this error:

file = models.FileField(upload_to=UploadItem.get_directory) 
NameError: name 'UploadItem' is not defined

Answer 1

The error is natural given that at the time of evaluating

file = models.FileField(upload_to=UploadItem.get_directory) 

the UploadItem class is not yet defined. You can do the following to make it work:

def get_directory():
    pass

class UploadItem(models.Model):
    file = models.FileField(upload_to=get_directory)

    class Meta:
        abstract = True

This won't solve all your problems though. Adding (or overriding) a method get_directory in the Video class will not change the upload_to property of the file attribute of the model.

Update

The documentation says that the upload_to can be a callable.

This may also be a callable, such as a function, which will be called to obtain the upload path, including the filename. This callable must be able to accept two arguments, and return a Unix-style path (with forward slashes) to be passed along to the storage system.

Given this we can write a custom call back function like this:

categories_and_paths = { 'video': 'videos/', 'photo': 'photos/' } # etc.
def get_directory(instance, filename):
    category = instance.category
    return categories_and_paths.get(category, '')

Instance here will be the instance of the respective model. For this to work each model instance should have a category field. We can add one in the body of the model.

class Video(UploadItem):
    category = 'video'