Django condition in save method

I got storage places which have the field amount and article. If I call save() on one place I want to set the article field to None. What is the best approach for this?

I already tried this:

    def save(self, *args, **kwargs):
    if self.amount == 0:
        self.article = None
    super(LagerPlace, self).save(*args, **kwargs)

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