我什么时候需要使用sqlalchemy back_populates？

Question

当我按照本指南尝试SQLAlchemy Relation Example时:基本关系模式

我有这个代码

#!/usr/bin/env python
# encoding: utf-8
from sqlalchemy import create_engine
from sqlalchemy import Table, Column, Integer, ForeignKey
from sqlalchemy.orm import relationship, sessionmaker
from sqlalchemy.ext.declarative import declarative_base

engine = create_engine('sqlite:///:memory:', echo=True)
Session = sessionmaker(bind=engine)
session = Session()
Base = declarative_base(bind=engine)

class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    children = relationship("Child")

class Child(Base):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)
    parent_id = Column(Integer, ForeignKey('parent.id'))
    parent = relationship("Parent")

Base.metadata.create_all()

p = Parent()
session.add(p)
session.commit()
c = Child(parent_id=p.id)
session.add(c)
session.commit()
print "children: {}".format(p.children[0].id)
print "parent: {}".format(c.parent.id)

它运作良好,但在指南中,它说模型应该是:

class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    **children = relationship("Child", back_populates="parent")**

class Child(Base):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)
    parent_id = Column(Integer, ForeignKey('parent.id'))
    **parent = relationship("Parent", back_populates="children")**

为什么我不需要back_populates或backref在我的例子中？我什么时候应该使用其中一种？

Answer 1

如果您使用backref,则无需在第二个表上声明关系.

class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    children = relationship("Child", backref="parent")

class Child(Base):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)
    parent_id = Column(Integer, ForeignKey('parent.id'))

如果您没有使用backref,并且relationship单独定义,那么如果您不使用back_populates,sqlalchemy将不知道连接关系,因此修改一个也会修改另一个.

因此,在您的示例中,您已经relationship单独定义了s,但没有提供back_populates参数,修改一个字段不会自动更新事务中的另一个字段.

>>> parent = Parent()
>>> child = Child()
>>> child.parent = parent
>>> print parent.children
[]

看看它是如何不自动填写的children？

现在,如果您提供back_populates参数,sqlalchemy将连接这些字段.

class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    children = relationship("Child", back_populates="parent")

class Child(Base):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)
    parent_id = Column(Integer, ForeignKey('parent.id'))
    parent = relationship("Parent", back_populates="children")

所以现在我们得到了

>>> parent = Parent()
>>> child = Child()
>>> child.parent = parent
>>> print parent.children
[Child(...)]

Sqlalchemy知道这两个字段现在是相关的,并且会在另一个更新时更新每个字段.值得注意的是,使用backref也会这样做.back_populates如果你想在每个类上定义关系,那么使用是很好的,所以很容易看到所有的字段都只是看了一下模型类,而不必查看通过backref定义字段的其他类.