我正在尝试将行从一个数据库实例复制到另一个数据库，该数据库在不同环境中具有相同的架构。此架构中的两个表以这样的方式链接：它们导致相互依赖的行。插入这些行后，post_update将按预期顺序运行，但是update语句会将ID字段的值设置为None而不是预期的ID。
当使用从会话中删除的对象时，仅发生 。使用新创建的对象时，post_update的行为完全符合预期。
示例
我建立了一个看起来像这样的关系：
class Category(Base):
__tablename__ = 'categories'
id = Column(Integer, primary_key=True)
top_product_id = Column(Integer, ForeignKey('products.id'))
products = relationship('Product', primaryjoin='Product.category_id == Category.id', back_populates='category', cascade='all', lazy='selectin')
top_product = relationship('Product', primaryjoin='Category.top_product_id == Product.id', post_update=True, cascade='all', lazy='selectin')
class Product(Base):
__tablename__ = 'products'
id = Column(Integer, primary_key=True)
category_id = Column(Integer, ForeignKey('categories.id'))
category = relationship('Category', primaryjoin='Product.category_id == Category.id', back_populates='products', cascade='all', lazy='selectin')
如果我从一个数据库查询类别及其相关产品，然后尝试将其写入另一个数据库，则top_product_id的更新将无法正常工作，而是将其值设置为None。以下代码：
category = source_session.query(Category).filter(Category.id == 99).one()
source_session.expunge(category)
make_transient(category)
for products in category.products:
make_transient(product)
# this step is necessary to prevent a foreign key error on the initial category insert
category.top_product_id = None
dest_session.add(category)
SQLAlchemy的结果将生成以下SQL：
INSERT INTO categories (name, top_product_id) VALUES (%s, %s)
('SomeCategoryName', None)
INSERT INTO products (name, category_id) VALUES (%s, %s)
('SomeProductName', 99)
UPDATE categories SET top_product_id=%s WHERE categories.id = %s
(None, 99)
但是，如果我使用新创建的对象，一切都会按预期进行。
category = Category()
product = Product()
category.name = 'SomeCategoryName'
product.name = 'SomeProductName'
product.category = category
category.top_product = product
dest_session.add(category)
导致：
INSERT INTO categories (name, top_product_id) VALUES (%s, %s)
('SomeCategoryName', None)
INSERT INTO products (name, category_id) VALUES (%s, %s)
('SomeProductName', 99)
UPDATE categories SET top_product_id=%s WHERE categories.id = %s
(1, 99)
除了这种区别外，这两个动作之间的所有行为都相同。正确创建了所有其他关系，并按预期设置了ID和外键。只有在top_product_id创建的update子句中设置的post_update无法正常工作。
作为其他故障排除步骤，我尝试了：
在第一次刷新到数据库时，top_product_id的设置正确。第二个参数设置为None。因此，这证实了问题不在于会话之间的差异，而是与从会话中删除对象并使它们成为临时对象有关。在删除/制作瞬态过程中，一定有/没有/没有发生某些事情，这些事情使这些对象处于根本不同的状态，并阻止post_update采取应有的方式。
任何关于从这里出发的想法都会受到赞赏。
答案 0 :(得分：1)
我假设您的Base类在name列中混在一起？
您的目标是使inspect(category).committed_state看起来像新创建的对象一样（也许id属性除外）。每个产品对象都相同。
在您的“新建对象”示例中，category的{{1}}在刷新会话之前如下所示：
committed_state {'id': symbol('NEVER_SET'),
'name': symbol('NO_VALUE'),
'products': [],
'top_product': symbol('NEVER_SET')}
的{{1}}看起来像这样：
product要获取更新后的行为，您既需要使committed_state过期（以防止其被包含在{'category': symbol('NEVER_SET'),
'id': symbol('NEVER_SET'),
'name': symbol('NO_VALUE')}
中），又要捏造category.top_product_id的{{1}} （使SQLAlchemy相信该值已更改，因此需要引起INSERT。）
首先，在category.top_product过渡之前使committed_state过期：
UPDATE然后捏造category.top_product_id的{{1}}（这可以在使category瞬变之前或之后发生）：
source_session.expire(category, ["top_product_id"])
完整示例：
category.top_product哪个在committed_state中生成此DML：
category似乎from sqlalchemy import inspect
from sqlalchemy.orm.base import NEVER_SET
inspect(category).committed_state.update(top_product=NEVER_SET)
应该将from sqlalchemy import Column, ForeignKey, Integer, String, create_engine, inspect
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import Session, make_transient, relationship
from sqlalchemy.orm.base import NEVER_SET
class Base(object):
name = Column(String(50), nullable=False)
Base = declarative_base(cls=Base)
class Category(Base):
__tablename__ = 'categories'
id = Column(Integer, primary_key=True)
top_product_id = Column(Integer, ForeignKey('products.id'))
products = relationship('Product', primaryjoin='Product.category_id == Category.id', back_populates='category', cascade='all', lazy='selectin')
top_product = relationship('Product', primaryjoin='Category.top_product_id == Product.id', post_update=True, cascade='all', lazy='selectin')
class Product(Base):
__tablename__ = 'products'
id = Column(Integer, primary_key=True)
category_id = Column(Integer, ForeignKey('categories.id'), nullable=False)
category = relationship('Category', primaryjoin='Product.category_id == Category.id', back_populates='products', cascade='all', lazy='selectin')
source_engine = create_engine('sqlite:///')
dest_engine = create_engine('sqlite:///', echo=True)
def fk_pragma_on_connect(dbapi_con, con_record):
dbapi_con.execute('pragma foreign_keys=ON')
from sqlalchemy import event
for engine in [source_engine, dest_engine]:
event.listen(engine, 'connect', fk_pragma_on_connect)
Base.metadata.create_all(bind=source_engine)
Base.metadata.create_all(bind=dest_engine)
source_session = Session(bind=source_engine)
dest_session = Session(bind=dest_engine)
source_category = Category(id=99, name='SomeCategoryName')
source_product = Product(category=source_category, id=100, name='SomeProductName')
source_category.top_product = source_product
source_session.add(source_category)
source_session.commit()
source_session.close()
# If you want to test UPSERTs in dest_session.
# dest_category = Category(id=99, name='PrevCategoryName')
# dest_product = Product(category=dest_category, id=100, name='PrevProductName')
# dest_category.top_product = dest_product
# dest_session.add(dest_category)
# dest_session.commit()
# dest_session.close()
category = source_session.query(Category).filter(Category.id == 99).one()
# Ensure relationship attributes are initialized before we make objects transient.
_ = category.top_product
# source_session.expire(category, ['id']) # only if you want new IDs in dest_session
source_session.expire(category, ['top_product_id'])
for product in category.products:
# Ensure relationship attributes are initialized before we make objects transient.
_ = product.category
# source_session.expire(product, ['id']) # only if you want new IDs in dest_session
# Not strictly needed as long as Product.category is not a post-update relationship.
source_session.expire(product, ['category_id'])
make_transient(category)
inspect(category).committed_state.update(top_product=NEVER_SET)
for product in category.products:
make_transient(product)
# Not strictly needed as long as Product.category is not a post-update relationship.
inspect(product).committed_state.update(category=NEVER_SET)
dest_session.add(category)
# Or, if you want UPSERT (must retain original IDs in this case)
# dest_session.merge(category)
dest_session.flush()
重置为新对象，但我想不是。