def get_last_tid(self):
        try:
            spec = {'tid': {'$gt': 0}}
            sort = [('tid', -1)]
            row = self.db.docs.find_one(spec, sort=sort)
        except Exception as e:
            raise DatabaseException(e)
        if row:
            return row['tid']
        return 0
def test_hyperfocal_distance():
    f, N, c = symbols('f, N, c')
    assert hyperfocal_distance(f=f, N=N, c=c) == f**2/(N*c)
    assert ae(hyperfocal_distance(f=0.5, N=8, c=0.0033), 9.47, 2)