<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def clear_last_check(self):
 """Clear the checksum of the file."""

        with db.session.begin_nested():
            self.last_check = None
            self.last_check_at = datetime.utcnow()
        return self