python - 如果在 SQLAlchemy 中将 "parent"对象分配给 "child"，如何在 "relation"对象上设置属性？

Question

当我有两个对象与 SQLAlchemy 中的“关系”相关联时，我意识到仅分配给该关系不足以将值传播到另一个对象。例如(见下文)，如果我有一个“用户”表和一个“联系人”表(两者都是高度人为的，但很好地证明了问题)，并且一个“用户”可以有多个“联系人”。在这种情况下，我将在用户和联系人之间使用外键。如果我同时创建 User 的实例和 Contact稍后将用户分配给联系人，我希望 FK 的属性会更新(即使没有数据库刷新)，但事实并非如此。为什么？我如何告诉 SA 自动执行此操作？

这将是我期望的工作，但正如您在下面的完整示例中看到的那样，它没有:

user = User(name='a', lname='b')
contact(type='email', value='foo@bar.com')
contact.user = user
assert contact.username == 'a'  # <-- Fails because the attribute is still `None`

完整的可运行示例:

"""
This code example shows two tables related to each other by a composite key,
using an SQLAlchemy "relation" to allow easy access to related items.

However, as the last few lines show, simply assigning an object A to the
relation of object B does not update the attributes of object B until at least
a "flush" is called.
"""
from sqlalchemy import Column, ForeignKeyConstraint, Unicode, create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relation, sessionmaker

Base = declarative_base()

class User(Base):
    __tablename__ = "user"

    name = Column(Unicode, primary_key=True)
    lname = Column(Unicode, primary_key=True)


class Contact(Base):
    __tablename__ = "contact"
    __table_args__ = (
        ForeignKeyConstraint(
            ['username', 'userlname'],
            ['user.name', 'user.lname']
        ),
    )

    username = Column(Unicode, primary_key=True)
    userlname = Column(Unicode, primary_key=True)
    type = Column(Unicode)
    value = Column(Unicode)

    user = relation(User)


engine = create_engine('sqlite://')
Base.metadata.create_all(engine)

Session = sessionmaker(bind=engine)
session = Session()

user = User(name="John", lname="Doe")
contact = Contact(type='email', value='john.doe@example.com')
contact.user = user  # <-- How can I tell SA to set the FKs on *contact* here?
session.add(contact)

print('Before flush: contact.username user=%r' % contact.username)
session.flush()
print('After flush : contact.username user=%r' % contact.username)

Answer 1

根据这个答案 - https://stackoverflow.com/a/52911047/4981223这不可能:

The FK of the child object isn't updated until you issue a flush() either explicitly or through a commit(). I think the reason for this is that if the parent object of a relationship is also a new instance with an auto-increment PK, SQLAlchemy needs to get the PK from the database before it can update the FK on the child object (but I stand to be corrected!).