我有一个sqlite3数据库，我正在使用Pony orm（版本0.7.1，使用python 3.6.0）访问。
其中一列/字段是持续时间（timedelta）。我正在尝试选择持续时间少于给定时间的所有行。
我正在做的最简单的例子：
import pony.orm
from datetime import timedelta
db = pony.orm.Database()
class TestTime(db.Entity):
id = pony.orm.PrimaryKey(int, auto=True)
td = pony.orm.Required(timedelta)
db.bind('sqlite', ':memory:', create_db=True)
db.generate_mapping(create_tables=True)
if __name__ == "__main__":
with pony.orm.db_session:
tt1 = TestTime(td=timedelta(seconds=12))
tt2 = TestTime(td=timedelta(seconds=18))
db.commit()
delta = timedelta(seconds=15)
with pony.orm.db_session:
TestTime.select(lambda x: x.td < delta)
当我运行时，我得到AssertionError: delta
使用pony.orm.select(x for x in TestTime if x.td < delta)语法进行查询会产生相同的错误。
完全跟踪是否有帮助：
Traceback (most recent call last):
File "ponytd.py", line 23, in <module>
TestTime.select(lambda x: x.td < delta)
File "<string>", line 2, in select
File "/home/user/.local/share/virtualenvs/test-Tqz3IJV7/lib/python3.6/site-packages/pony/utils/utils.py", line 58, in cut_traceback
return func(*args, **kwargs)
File "/home/user/.local/share/virtualenvs/test-Tqz3IJV7/lib/python3.6/site-packages/pony/orm/core.py", line 3596, in select
return entity._query_from_args_(args, kwargs=None, frame_depth=3)
File "/home/user/.local/share/virtualenvs/test-Tqz3IJV7/lib/python3.6/site-packages/pony/orm/core.py", line 3943, in _query_from_args_
return Query(code_key, inner_expr, globals, locals, cells)
File "/home/user/.local/share/virtualenvs/test-Tqz3IJV7/lib/python3.6/site-packages/pony/orm/core.py", line 5087, in __init__
translator = translator_cls(tree, extractors, vartypes, left_join=left_join)
File "/home/user/.local/share/virtualenvs/test-Tqz3IJV7/lib/python3.6/site-packages/pony/orm/sqltranslation.py", line 269, in __init__
translator.dispatch(if_)
File "/home/user/.local/share/virtualenvs/test-Tqz3IJV7/lib/python3.6/site-packages/pony/orm/sqltranslation.py", line 82, in dispatch
return ASTTranslator.dispatch(translator, node) # default route
File "/home/user/.local/share/virtualenvs/test-Tqz3IJV7/lib/python3.6/site-packages/pony/orm/asttranslation.py", line 29, in dispatch
translator.dispatch(child)
File "/home/user/.local/share/virtualenvs/test-Tqz3IJV7/lib/python3.6/site-packages/pony/orm/sqltranslation.py", line 82, in dispatch
return ASTTranslator.dispatch(translator, node) # default route
File "/home/user/.local/share/virtualenvs/test-Tqz3IJV7/lib/python3.6/site-packages/pony/orm/asttranslation.py", line 24, in dispatch
stop = translator.call(pre_method, node)
File "/home/user/.local/share/virtualenvs/test-Tqz3IJV7/lib/python3.6/site-packages/pony/orm/sqltranslation.py", line 135, in call
reraise(exc_class, exc, tb)
File "/home/user/.local/share/virtualenvs/test-Tqz3IJV7/lib/python3.6/site-packages/pony/utils/utils.py", line 85, in reraise
try: raise exc.with_traceback(tb)
File "/home/user/.local/share/virtualenvs/test-Tqz3IJV7/lib/python3.6/site-packages/pony/orm/sqltranslation.py", line 125, in call
try: monad = method(node)
File "/home/user/.local/share/virtualenvs/test-Tqz3IJV7/lib/python3.6/site-packages/pony/orm/sqltranslation.py", line 649, in preCompare
translator.dispatch(right)
File "/home/user/.local/share/virtualenvs/test-Tqz3IJV7/lib/python3.6/site-packages/pony/orm/sqltranslation.py", line 83, in dispatch
translator.call(translator.dispatch_external, node)
File "/home/user/.local/share/virtualenvs/test-Tqz3IJV7/lib/python3.6/site-packages/pony/orm/sqltranslation.py", line 135, in call
reraise(exc_class, exc, tb)
File "/home/user/.local/share/virtualenvs/test-Tqz3IJV7/lib/python3.6/site-packages/pony/utils/utils.py", line 85, in reraise
try: raise exc.with_traceback(tb)
File "/home/user/.local/share/virtualenvs/test-Tqz3IJV7/lib/python3.6/site-packages/pony/orm/sqltranslation.py", line 125, in call
try: monad = method(node)
File "/home/user/.local/share/virtualenvs/test-Tqz3IJV7/lib/python3.6/site-packages/pony/orm/sqltranslation.py", line 119, in dispatch_external
monad = translator.ParamMonad.new(translator, t, (varkey, None, None))
File "/home/user/.local/share/virtualenvs/test-Tqz3IJV7/lib/python3.6/site-packages/pony/orm/sqltranslation.py", line 1646, in new
result = cls(translator, type, paramkey)
File "/home/user/.local/share/virtualenvs/test-Tqz3IJV7/lib/python3.6/site-packages/pony/orm/sqltranslation.py", line 1654, in __init__
Monad.__init__(monad, translator, type)
File "/home/user/.local/share/virtualenvs/test-Tqz3IJV7/lib/python3.6/site-packages/pony/orm/sqltranslation.py", line 1006, in __init__
monad.mixin_init()
File "/home/user/.local/share/virtualenvs/test-Tqz3IJV7/lib/python3.6/site-packages/pony/orm/sqltranslation.py", line 986, in wrapper
return method(monad, *args, **kwargs)
File "/home/user/.local/share/virtualenvs/test-Tqz3IJV7/lib/python3.6/site-packages/pony/orm/sqltranslation.py", line 1310, in mixin_init
assert monad.type is time
AssertionError: delta
我做错了什么，或者有更好的方法吗？