Python 为什么查询会在 SQLAlchemy 中调用自动刷新？
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Why does a query invoke a auto-flush in SQLAlchemy?
提问by buhtz
The code you see above is just a sample but it works to reproduce this error:
您在上面看到的代码只是一个示例，但它可以重现此错误：
sqlalchemy.exc.IntegrityError: (raised as a result of Query-invoked autoflush;
consider using a session.no_autoflush block if this flush is occurring prematurely)
(sqlite3.IntegrityError) NOT NULL constraint failed: X.nn
[SQL: 'INSERT INTO "X" (nn, val) VALUES (?, ?)'] [parameters: (None, 1)]
A mapped instance is still added to a session. The instance wants to know (which means query on the database) if other instances its own type exists having the same values. There is a second attribute/column (_nn). It is specified to NOT NULL. But by default it is NULL.
映射实例仍会添加到会话中。该实例想知道（这意味着在数据库上查询）是否存在其自己类型具有相同值的其他实例。还有第二个属性/列 ( _nn)。它被指定为NOT NULL。但默认情况下它是NULL.
When the instance (like in the sample) is still added to the session a call to query.one()invoke a auto-flush. This flush create an INSERTwhich tries to store the instance. This fails because _nnis still null and violates the NOT NULLconstraint.
当实例（如示例中）仍被添加到会话时，query.one()调用自动刷新。此刷新创建一个INSERT尝试存储实例的对象。这将失败，因为_nn它仍然为 null 并且违反了NOT NULL约束。
That is what I understand currently. But the question is why does it invoke an auto-flush? Can I block that?
这是我目前的理解。但问题是为什么它会调用自动刷新？我可以阻止吗？
#!/usr/bin/env python3
import os.path
import os
import sqlalchemy as sa
import sqlalchemy.orm as sao
import sqlalchemy.ext.declarative as sad
from sqlalchemy_utils import create_database
_Base = sad.declarative_base()
session = None
class X(_Base):
__tablename__ = 'X'
_oid = sa.Column('oid', sa.Integer, primary_key=True)
_nn = sa.Column('nn', sa.Integer, nullable=False) # NOT NULL!
_val = sa.Column('val', sa.Integer)
def __init__(self, val):
self._val = val
def test(self, session):
q = session.query(X).filter(X._val == self._val)
x = q.one()
print('x={}'.format(x))
dbfile = 'x.db'
def _create_database():
if os.path.exists(dbfile):
os.remove(dbfile)
engine = sa.create_engine('sqlite:///{}'.format(dbfile), echo=True)
create_database(engine.url)
_Base.metadata.create_all(engine)
return sao.sessionmaker(bind=engine)()
if __name__ == '__main__':
session = _create_database()
for val in range(3):
x = X(val)
x._nn = 0
session.add(x)
session.commit()
x = X(1)
session.add(x)
x.test(session)
Of course a solution would be to notadd the instance to the session before query.one()was called. This work. But in my real (but to complex for this question) use-case it isn't a nice solution.
当然，解决方案是在调用之前不将实例添加到会话query.one()中。这项工作。但是在我真实的（但对于这个问题来说很复杂）用例中，这不是一个很好的解决方案。
采纳答案by Palasaty
How to turn off autoflushfeature:
如何关闭自动刷新功能：
Temporary: you can use no_autoflushcontext manager on snippet where you query the database, i.e. in
X.testmethod:def test(self, session): with session.no_autoflush: q = session.query(X).filter(X._val == self._val) x = q.one() print('x={}'.format(x))Session-wide: just pass
autoflush=Falseto your sessionmaker:return sao.sessionmaker(bind=engine, autoflush=False)()
临时：您可以在查询数据库的代码段上使用no_autoflush上下文管理器，即在
X.test方法中：def test(self, session): with session.no_autoflush: q = session.query(X).filter(X._val == self._val) x = q.one() print('x={}'.format(x))会话范围：只需传递
autoflush=False给您的会话创建者：return sao.sessionmaker(bind=engine, autoflush=False)()
