Django: How to Use upload_to=function with ModelForm

Question

The goal is to dynamically update upload_to such that user uploaded files are stored in a directory location that depends on the user. There are several examples of this online, but none using ModelForm. See the code snippets for the two problems, one is that I am getting an empty string for the instance.user value, and when I try and fix that, the form is not valid.

# models.py

def get_file_path( instance, filename ):
    # make the filepath include the signed in username
    print "inst: %s" % instance.__dict__.keys()
    print "inst:user:%s" % instance.user   # <-- This is empty string!!
    print "file: %s" % filename
    return "%s/myapp/%s/%s" % ( settings.MEDIA_ROOT, instance.user, filename )

class trimrcUpload(models.Model):
    user = models.CharField( max_length = 20 )
    inputFile = models.FileField( upload_to = get_file_path )


# forms. py

class trimrcUploadForm(ModelForm):

    class Meta:
        model = trimrcUpload
        exclude = ( 'resultFile', 'numTimesProcessed' )

# views.py

def myapp_upload( request, username, template_name="myapp/myapptemplate.html" ):

    dummy = trimrcUpload( user=username )
    if request.POST:
        form = trimrcUploadForm( request.POST, request.FILES, instance=dummy )
        if form.is_valid():
            success = form.save()
            success.save()

    # form is not valid, user is field is "required"
    # the user field is not displayed in the template by design,
    # it is to be populated by the view (above).

# http://docs.djangoproject.com/en/1.0/topics/forms/modelforms/
# about halfway down there is a "Note" section describing the use of dummy.

Answer 1

I would imagine your problem comes from trying to populate the user attribute of your model with a username. If the upload form will always be used by a logged in user, you can use this instead:

dummy = trimrcUpload( user=request.user )

Otherwise, if you still want to pass in the username like you do now, you can try something like:

try:
    user = User.objects.get(username=username)
    dummy = trimrcUpload( user=user )
except User.DoesNotExist:
    # Probably have to set some kind of form error

I would recommend going with the first option which would allow you to not have to pass username to the view.

Answer 2

The original code in the question actually works.