我正在尝试使用SQLAlchemy automap扩展为现有数据库生成ORM，并且收到InvalidRequestError异常（“实例无法刷新 - 它不是持久性的，并且不包含完整的主键。”）每当我尝试插入一个使用由时间戳和外键组成的复合主键的表。
以下是一些重现问题的最小示例代码：
from sqlalchemy import create_engine, func, select
from sqlalchemy.orm import sessionmaker
from sqlalchemy.sql.expression import text
from sqlalchemy.ext.automap import automap_base
db_schema_cmds = [
'''CREATE TABLE users
(
u_id INTEGER NOT NULL,
name TEXT NOT NULL,
CONSTRAINT Key1 PRIMARY KEY (u_id)
);''',
'''CREATE TABLE posts
(
timestamp TEXT NOT NULL,
text TEXT NOT NULL,
u_id INTEGER NOT NULL,
CONSTRAINT Key2 PRIMARY KEY (timestamp,u_id),
CONSTRAINT users_have_posts FOREIGN KEY (u_id) REFERENCES users (u_id) ON DELETE CASCADE
);''']
# Create a new in-memory SQLite DB and execute the schema SQL commands.
db_engine = create_engine('sqlite://')
with db_engine.connect() as db_conn:
for cmd in db_schema_cmds:
db_conn.execute(text(cmd))
# Use automap to reflect the DB schema and generate ORM classes.
Base = automap_base()
Base.prepare(db_engine, reflect=True)
# Create aliases for the table classes generated.
User = Base.classes.users
Post = Base.classes.posts
session_factory = sessionmaker()
session_factory.configure(bind=db_engine)
# Add a user and a post to the DB.
session = session_factory()
new_user = User(name="John")
session.add(new_user)
session.commit()
new_post = Post(users=new_user, text='this is a test', timestamp=func.now())
session.add(new_post)
session.commit()
# Verify that the insertion worked.
new_user_id = session.execute(select([User])).fetchone()['u_id']
new_post_fk_user_id = session.execute(select([Post])).fetchone()['u_id']
assert new_user_id == new_post_fk_user_id
session.close()
运行此命令会产生以下追溯：
Traceback (most recent call last):
File "reproduce_InvalidRequestError.py", line 67, in <module>
session.commit()
File "C:\Python\Python35\lib\site-packages\sqlalchemy\orm\session.py", line 801, in commit
self.transaction.commit()
File "C:\Python\Python35\lib\site-packages\sqlalchemy\orm\session.py", line 392, in commit
self._prepare_impl()
File "C:\Python\Python35\lib\site-packages\sqlalchemy\orm\session.py", line 372, in _prepare_impl
self.session.flush()
File "C:\Python\Python35\lib\site-packages\sqlalchemy\orm\session.py", line 2019, in flush
self._flush(objects)
File "C:\Python\Python35\lib\site-packages\sqlalchemy\orm\session.py", line 2137, in _flush
transaction.rollback(_capture_exception=True)
File "C:\Python\Python35\lib\site-packages\sqlalchemy\util\langhelpers.py", line 60, in __exit__
compat.reraise(exc_type, exc_value, exc_tb)
File "C:\Python\Python35\lib\site-packages\sqlalchemy\util\compat.py", line 186, in reraise
raise value
File "C:\Python\Python35\lib\site-packages\sqlalchemy\orm\session.py", line 2107, in _flush
flush_context.finalize_flush_changes()
File "C:\Python\Python35\lib\site-packages\sqlalchemy\orm\unitofwork.py", line 395, in finalize_flush_changes
self.session._register_newly_persistent(other)
File "C:\Python\Python35\lib\site-packages\sqlalchemy\orm\session.py", line 1510, in _register_newly_persistent
instance_key = mapper._identity_key_from_state(state)
File "C:\Python\Python35\lib\site-packages\sqlalchemy\orm\mapper.py", line 2417, in _identity_key_from_state
for col in self.primary_key
File "C:\Python\Python35\lib\site-packages\sqlalchemy\orm\mapper.py", line 2417, in <listcomp>
for col in self.primary_key
File "C:\Python\Python35\lib\site-packages\sqlalchemy\orm\attributes.py", line 578, in get
value = state._load_expired(state, passive)
File "C:\Python\Python35\lib\site-packages\sqlalchemy\orm\state.py", line 474, in _load_expired
self.manager.deferred_scalar_loader(self, toload)
File "C:\Python\Python35\lib\site-packages\sqlalchemy\orm\loading.py", line 647, in load_scalar_attributes
"contain a full primary key." % state_str(state))
sqlalchemy.exc.InvalidRequestError: Instance <posts at 0x45d45f8> cannot be refreshed - it's not persistent and does not contain a full primary key.
如果我在echo=True调用中添加了create_engine参数，我看到它正在为插入生成以下SQL。当我在DB Browser for SQLite中运行它时，这个SQL工作正常。
INFO sqlalchemy.engine.base.Engine BEGIN (implicit)
INFO sqlalchemy.engine.base.Engine SELECT users.u_id AS users_u_id, users.name AS users_name FROM users WHERE users.u_id = ?
INFO sqlalchemy.engine.base.Engine (1,)
INFO sqlalchemy.engine.base.Engine INSERT INTO posts (timestamp, text, u_id) VALUES (CURRENT_TIMESTAMP, ?, ?)
INFO sqlalchemy.engine.base.Engine ('this is a test', 1)
INFO sqlalchemy.engine.base.Engine ROLLBACK
我还尝试从users删除Post()参数，然后在调用new_user.posts_collection.append(new_post)之前添加行session.add(new_post)，但这导致生成相同的SQL并且相同错误发生。
如果我用新的整数PK列替换复合键，一切正常。 （虽然不是一个理想的解决方案，因为我使用automap的原因是反映现有的数据库，因此最好不要修改该数据库的模式。）
我发现了一个类似的问题SQLAlchemy InvalidRequestError when using composite foreign keys，但这似乎与在表ORM类中使用继承有关，而解决方案依赖于定义ORM表类，而不是反映数据库生成它们。
编辑：我原先认为这个问题与我的复合主键包含外键这一事实有关。接受的答案显示，外键实际上并不是解决问题的因素。
答案 0 :(得分：2)
问题实际上不是具有外键的复合主键，而func.now()传递为timestamp，它是主键的一部分。由于SQLAlchemy不知道该值，因为它是在数据库中插入期间生成的，因此无法执行post-fetch;它不知道要取什么。如果相关数据库支持RETURNING或类似，您就可以执行此操作。请参阅triggered columns上的说明，其中介绍了这种情况。 Defaults / SQL Expressions还介绍了主键值的预执行SQL。
它使用整数代理主键的原因是SQLite确实有一个fetching the last inserted row id（一个整数主键列）的机制，SQLAlchemy可以使用它。
要解决此问题，您可以使用Python中生成的时间戳
In [8]: new_post = Post(users=new_user, text='this is a test',
...: timestamp=datetime.utcnow())
...: session.add(new_post)
...: session.commit()
...:
另一种解决方案是在反射期间覆盖timestamp列，并提供func.now()作为默认值。这将触发func.now()的预执行。
...: # Use automap to reflect the DB schema and generate ORM classes.
...: Base = automap_base()
...:
...: # Override timestamp column before reflection
...: class Post(Base):
...: __tablename__ = 'posts'
...: timestamp = Column(Text, nullable=False, primary_key=True,
...: default=func.now())
...:
...: Base.prepare(db_engine, reflect=True)
...:
...: # Create aliases for the table classes generated.
...: User = Base.classes.users
...: # Post has already been declared
...: #Post = Base.classes.posts
默认情况下，您在创建新实例时不需要（也不应该）提供timestamp
In [6]: new_post = Post(users=new_user, text='this is a test')
...: session.add(new_post)
...: session.commit()
...: