def delete(self, doc):
        if doc.id is None:
            raise DatabaseException('Document does not have id')
        if doc.sid is None:
            try:
                rs = self.db.docs.remove({'id': doc.id}, safe=True)
                assert rs['n'] == 1
            except Exception as e:
                raise DatabaseException(e)
            return None
        doc.data = None
        return self.update(doc)
def days_till_due(self):
        """
        Returns the number of days till the due date. Returns a negative number
        of days when the due date is in the past.
        Returns 0 when the task has no due date.
        """
        due = self.due_date()
        if due:
            diff = due - date.today()
            return diff.days
        return 0