The sample space is large 2 ** (8 * 20), but the probability is not 0 that generate_key returns a key that already exists, and then the save would update the existing one.

The smallest change, turns this into an integrity error:

     def save(self, *args, **kwargs): 
         if not self.key: 
             self.key = self.generate_key()
             kwargs["force_insert"] = True
         return super().save(*args, **kwargs)