当实例分配给关系时，sqlalchemy before_flush 事件处理程序看不到外键的变化

Question

我有一个before_flush事件侦听器，用于检查员工的经理是否更改。在这种情况下，会在EmpManHist表中自动创建记录。manager是对Employee表的自引用。这是我的表定义：

class Employee(Base):
    __tablename__ = 'employees'

    emp_id = Column(String, primary_key=True, unique=True)
    name = Column(String, nullable=False)
    manager_id = Column(String, ForeignKey('employees.emp_id'))
    direct_reports = relationship('Employee', backref=backref('manager', remote_side=[emp_id]))

当我通过manager_id直接修改 ForeignKey ( )将新经理分配给员工时，它会被事件侦听器正确拾取。但是当我进行实例分配时，它不会：

# this code does not pick up the manager_id change in the 'before_flush' event listener
emp2.manager = emp3
dal.session.add(emp2)
dal.session.commit()

# this does
emp2.manager_id = '1'
dal.session.add(emp2)
dal.session.commit()

这是为什么？我正在使用 python 3.6.3 和 sqlalchemy 1.1.13

以下是完整的工作示例：

from sqlalchemy import create_engine, Column, Integer, String, DateTime, ForeignKey
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker, relationship, backref
from sqlalchemy import event
from sqlalchemy.orm.attributes import get_history
import datetime


Base = declarative_base()


class DataAccessLayer(object):

    def __init__(self):
        self.conn_string = conn_string
        self.engine = None
        self.session = None
        self.Session = None
        self.echo = True

    def connect(self):
        self.engine = create_engine(self.conn_string, echo=self.echo)
        Base.metadata.create_all(self.engine)
        self.Session = sessionmaker(bind=self.engine)


class Employee(Base):
    __tablename__ = 'employees'

    emp_id = Column(String, primary_key=True, unique=True)
    name = Column(String, nullable=False)
    manager_id = Column(String, ForeignKey('employees.emp_id'))
    direct_reports = relationship('Employee', backref=backref('manager', remote_side=[emp_id]))


class EmpManHist(Base):
    __tablename__ = 'emp_man_history'

    id = Column(Integer, primary_key=True)
    emp_id = Column(String, ForeignKey('employees.emp_id'))
    man_id_from = Column(String, ForeignKey('employees.emp_id'))
    man_id_to = Column(String, ForeignKey('employees.emp_id'))
    when = Column(DateTime, default=datetime.datetime.now)

    manager_from = relationship('Employee', foreign_keys=[man_id_from])
    manager_to = relationship('Employee', foreign_keys=[man_id_to])


conn_string = 'sqlite:///:memory:'
dal = DataAccessLayer()
dal.echo = True
dal.connect()
dal.session = dal.Session()


@event.listens_for(dal.session, 'before_flush')
def _emp_history_update(session, flush_context, instances):
    print("BEFORE FLUSH")
    for instance in session.dirty:
        if not isinstance(instance, Employee):
            continue
        man_hist = get_history(instance, 'manager_id')
        if man_hist.added:
            if man_hist.deleted:
                man_deleted = str(man_hist.deleted[0])
            else:
                man_deleted = None
            emp_man_hist = EmpManHist(emp_id=instance.emp_id, man_id_from=man_deleted,
                                      man_id_to=str(man_hist.added[0]))
            session.add(emp_man_hist)


emp1 = Employee(emp_id='1', name="AAA")
emp2 = Employee(emp_id='2', name="BBB", manager_id='1')
emp3 = Employee(emp_id='3', name="CCC", manager_id='1')


dal.session.add(emp3)
dal.session.flush()
dal.session.add(emp1)
dal.session.add(emp2)

dal.session.commit()

# this code does not pick up the manager_id change in the 'before_flush' event listener
emp2.manager = emp3
dal.session.add(emp2)
dal.session.add(emp3)
dal.session.commit()

# this does
emp2.manager_id = '1'
dal.session.add(emp2)
dal.session.commit()

Answer 1

当关系属性更改时，SQLAlchemy 不会立即更新外键字段。所以你的问题的答案before_flush是在 SQLAlchemy 更新Employee实例的外键值之前触发事件作为flush操作的一部分。

在您manager_id自己专门更新属性的情况下，该属性before_flush在触发事件之前已更改，这就是_emp_history_update在这种情况下您会看到函数更改的原因。

您仍然可以使用该after_flush事件做您想做的任何事情，因为session.dirty到那时尚未清除。所以我将您的事件侦听器更改为：

@event.listens_for(session, 'after_flush')
def _emp_history_update(session, flush_context):
    for instance in session.dirty:
        if not isinstance(instance, Employee):
            continue
        man_hist = get_history(instance, 'manager_id')
        if man_hist.added:
            if man_hist.deleted:
                man_deleted = str(man_hist.deleted[0])
            else:
                man_deleted = None
            emp_man_hist = EmpManHist(emp_id=instance.emp_id, man_id_from=man_deleted,
                                      man_id_to=str(man_hist.added[0]))
            session.add(emp_man_hist)

这是测试代码：

emp1 = Employee(emp_id='1', name="AAA")
emp2 = Employee(emp_id='2', name="BBB", manager_id='1')
emp3 = Employee(emp_id='3', name="CCC", manager_id='1')

# I'm not using your DataAccessLayer object but that doesn't change anything
session.add_all([emp1, emp2, emp3])
# i've not explicitly called session.flush() as it's called by session.commit() anyway
session.commit()

# change the emp2's manager through relationship attribute
emp2.manager = emp3
# no need to re-add the Employee objects to the session
session.commit()

for change in session.query(EmpManHist).all():
    print(f'Employee {change.emp_id} changed to mgr_id {change.man_id_to}')

哪些输出：

Employee 2 changed to mgr_id 3

我注意到的另一件事是，在事件侦听器的这一部分中：

    if man_hist.added:
        if man_hist.deleted:
            man_deleted = str(man_hist.deleted[0])
        else:
            man_deleted = None
        emp_man_hist = EmpManHist(emp_id=instance.emp_id, man_id_from=man_deleted,
                                  man_id_to=str(man_hist.added[0]))
        session.add(emp_man_hist)

改变manager_id了的Employee，通过自己直接更改属性，或通过改变这两种情况下Employee.manager的关系属性，从不在一个实例展示man_hist.deleted。因此，您man_id_from的EmpManHist实例属性始终为None.

这是从您的示例代码生成的INSERTinto的日志，emp_man_hist您可以从第二行中的值列表中看到，与 对齐的第二个值man_id_from正在被分配None：

2018-07-27 09:03:41,189 INFO sqlalchemy.engine.base.Engine INSERT INTO emp_man_history (emp_id, man_id_from, man_id_to, "when") VALUES (?, ?, ?, ?)
2018-07-27 09:03:41,189 INFO sqlalchemy.engine.base.Engine ('2', None, '1', '2018-07-27 09:03:41.188906')