Django uses this 'force_insert' when you call Manager.create(). But here we uses the save method several times so we have to pop this kwargs, because the second run can not be an INSERT. The problem is shown es error about primary keys. I think it is a hack, because I do not know Django's reasons to uses this force_insert but I throw it simply away. Better would be not to misuse the save method to check the uniqueness of the identifier.