Question

下面是完整的工作示例。将manager分配给Employee的实例时，before_flush事件侦听器不会获取更改。但是，当我直接更改manager_id时，它确实会更改。这是为什么？我是不正确地处理事件，还是出于某种原因而设计的？

编辑此外，我刚刚意识到，不仅第一次提交不会触发向EmpManHist表的插入，而且在第二次提交EmpManHist显示从None到1的更改之后。我希望3至1

我正在使用python 3.6.3和sqlalchemy 1.1.13

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.commit()

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

分配给实例时，sqlalchemy before_flush不会获取属性更改

0 个答案: