python-使用AttributeExtension自动更新非标准化的属性

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我在使用SQLAlchemy的AttributeExtension时遇到了一些麻烦.

实际上,我将非规范化的sum属性存储在Partent表中,因为出于排序目的,我经常需要它.但是,我希望每当其中一个子项的值更改时,该属性即可更新.

不幸的是,从未调用AttributeExtension的set()方法,因此无法识别更改.使用可以同时更新父级的属性设置器可能会起作用,但是我想知道如何正确使用SQLAlchemy的AttributeExtension(版本：0.6beta2).

这是一个小的(可运行的)代码段,它演示了该问题：

from sqlalchemy import create_engine, Column, Integer, ForeignKey
from sqlalchemy.orm import relation, scoped_session, sessionmaker, \
         AttributeExtension
from sqlalchemy.ext.declarative import declarative_base

engine = create_engine('sqlite:///:memory:', echo=True)
session = scoped_session(sessionmaker(bind=engine, autoflush=True))
Base = declarative_base()
Base.query = session.query_property()

class ChildrenAttributeExtension(AttributeExtension):
    active_history = True

    def append(self, state, child, initiator):
        parent = state.obj()
        parent.sum_of_children += child.value
        return child

    def remove(self, state, child, initiator):
        parent = state.obj()
        parent.sum_of_children -= child.value

    def set(self, state, child, oldchild, initiator):
        print 'set called' # gets never printed
        parent = state.obj()
        parent.sum_of_children += -oldchild.value + child.value
        return child


class Child(Base):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)
    parent_id = Column(Integer, ForeignKey('parent.id'), nullable=False)
    value = Column(Integer, nullable=False, default=0)


class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    sum_of_children = Column(Integer, nullable=False, default=0)

    children = relation('Child', backref='parent',
            extension=ChildrenAttributeExtension())

Base.metadata.create_all(engine)

# Add a parent
p = Parent()
session.add(p)
session.commit()

p = Parent.query.first()
assert p.sum_of_children == 0


# Add a child
c = Child(parent=p, value=5)
session.add(c)
session.commit()

p = Parent.query.first()
assert p.sum_of_children == 5

# Change a child
c = Child.query.first()
c.value = 3
session.commit()  # extension.set() doesn't get called

p = Parent.query.first()
assert p.sum_of_children == 3 # Assertion fails

谢谢你的帮助！
克里斯多夫

解决方法:

据我所知,您正在寻找有关child的事件,但要更改child.value.这样的事情应该可以解决问题：

class ValueAttributeExtension(AttributeExtension):
  ...

class Child(Base):
  ...
  value = ColumnProperty(Column(Integer, nullable=False, default=0), 
                         extension=ValueAttributeExtension()) 

EDIT-1：完整的工作示例如下：

from sqlalchemy import create_engine, Column, Integer, ForeignKey
from sqlalchemy.orm import relation, scoped_session, sessionmaker, AttributeExtension, ColumnProperty
from sqlalchemy.ext.declarative import declarative_base

engine = create_engine('sqlite:///:memory:', echo=False)
session = scoped_session(sessionmaker(bind=engine, autoflush=True))
Base = declarative_base()
Base.query = session.query_property()

class ValueAttributeExtension(AttributeExtension):
    active_history = True

    def append(self, state, child, initiator):
        assert False, "should not be called"

    def remove(self, state, child, initiator):
        assert False, "should not be called"

    def set(self, state, value, oldvalue, initiator):
        print 'set called', state.obj(), value, oldvalue
        child = state.obj()
        if not(child.parent is None):
            child.parent.sum_of_children += -oldvalue + value
        return value

class ChildrenAttributeExtension(AttributeExtension):
    active_history = True

    def append(self, state, child, initiator):
        print 'append called', state.obj(), child
        parent = state.obj()
        parent.sum_of_children += child.value
        return child

    def remove(self, state, child, initiator):
        print 'remove called', state.obj(), child
        parent = state.obj()
        parent.sum_of_children -= child.value

    def set(self, state, child, oldchild, initiator):
        print 'set called', state, child, oldchild
        parent = state.obj()
        parent.parent.sum_of_children += -oldchild.value + child.value
        #parent.sum_of_children += -oldchild.value + child.value
        return child

class Child(Base):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)
    parent_id = Column(Integer, ForeignKey('parent.id'), nullable=False)
    value = ColumnProperty(Column(Integer, nullable=False, default=0),
                    extension=ValueAttributeExtension())

class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    sum_of_children = Column(Integer, nullable=False, default=0)

    children = relation('Child', backref='parent',
                        extension=ChildrenAttributeExtension())

Base.metadata.create_all(engine)

# Add a parent
p = Parent()
session.add(p)
session.commit()

p = Parent.query.first()
assert p.sum_of_children == 0


# Add a child
c = Child(parent=p, value=5)
session.add(c)
session.commit()

p = Parent.query.first()
assert p.sum_of_children == 5

# Change a child
#c = Child.query.first()
c.value = 3 # fixed bug: = instead of ==
session.commit()  # extension.set() doesn't get called

p = Parent.query.first()
assert p.sum_of_children == 3 # Assertion is OK

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