SlugModel: Avoid saving with force_insert twice

If the 'force_insert' flag was passed, don't pass it again:
doing so will attempt to re-insert with the same primary key,
which will cause an IntegrityError.

Author: PiDelport

cities/models.py

@@ -47,6 +47,11 @@ def save(self, *args, **kwargs):
                 self.slug = 'invalid-{}'.format(''.join(choice(ascii_uppercase + digits) for i in range(20)))
                 super(SlugModel, self).save(*args, **kwargs)
                 self.slug = slugify_func(self, self.slugify())
+
+                # If the 'force_insert' flag was passed, don't pass it again:
+                # doing so will attempt to re-insert with the same primary key,
+                # which will cause an IntegrityError.
+                kwargs.pop('force_insert', None)
                 super(SlugModel, self).save(*args, **kwargs)
         else:
             # This is a performance optimization - we avoid the transaction if