Question

在sqlalchemy中，我可以侦听集合上的append事件来拦截并可能更改要附加的值。我怎样才能静默地删除值（不附加），例如当我想用任意标准实现类似set的行为时？

MCVE（除了sqlalchemy之外没有任何依赖，只需复制和粘贴）：

from sqlalchemy import create_engine, Integer, Text

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship, sessionmaker
from sqlalchemy.schema import Column, ForeignKey

from sqlalchemy.event import listens_for

engine = create_engine('sqlite:///:memory:')
Base = declarative_base()

_next_id = 1


def get_id():
    global _next_id
    _ = _next_id
    _next_id += 1
    return _


class A1(Base):
    __tablename__ = 'a1'
    id = Column(Integer, primary_key=True, default=get_id)
    a2 = relationship('A2', back_populates='a')


class A2(Base):
    __tablename__ = 'a2'
    id = Column(Integer, primary_key=True, default=get_id)
    a_id = Column(Integer, ForeignKey('a1.id'))
    a = relationship('A1', back_populates='a2')

    name = Column(Text)


@listens_for(A1.a2, 'append', retval=True)
def coll_listener(target, val, initiator):
    for _ in target.a2:
        if _.name == val.name:
            # this is justs some arbitrary condition to illustrate things
            return
    return val


Base.metadata.create_all(engine)
Session = sessionmaker()
Session.configure(bind=engine)
_session = Session()

a2_1 = A2(name='a2_1')
a2_2 = A2(name='a2_2')
a2_3 = A2(name='a2_1')

a1 = A1()

for thing in (a2_1, a2_2, a2_3, a1):
    _session.add(thing)

_session.flush()

for a in (a2_1, a2_2, a2_3):
    a1.a2.append(a)

_session.flush()

# raises FlushError: Can't flush None value found in collection A1.a2

Answer 1

在这里回答自己，因为nobobdy感兴趣...

我现在相信，在添加带有事件监听器的时，无法删除元素。所以我最终实现了custom collection（这并不是完全鼓励的），结果证明是相当简单的：

from sqlalchemy import create_engine, Integer, Text from sqlalchemy.ext.declarative import declarative_base from sqlalchemy.orm import relationship, sessionmaker from sqlalchemy.schema import Column, ForeignKey engine = create_engine('sqlite:///:memory:') Base = declarative_base() _next_id = 1 def get_id(): global _next_id _ = _next_id _next_id += 1 return _ class NamedThingCollection(object): def __init__(self): self.data = [] def __contains__(self, thing): return next((True for x in self.data if x.name == thing.name), False) def append(self, item): if item not in self: self.data.append(item) def remove(self, item): self.data.remove(item) def extend(self, items): self.data.extend(items) def __iter__(self): return iter(self.data) def __len__(self): return len(self.data) class A1(Base): __tablename__ = 'a1' id = Column(Integer, primary_key=True, default=get_id) a2 = relationship( 'A2', back_populates='a', collection_class=NamedThingCollection) class A2(Base): __tablename__ = 'a2' id = Column(Integer, primary_key=True, default=get_id) a_id = Column(Integer, ForeignKey('a1.id')) a = relationship('A1', back_populates='a2') name = Column(Text) Base.metadata.create_all(engine) Session = sessionmaker() Session.configure(bind=engine) _session = Session() a2_1 = A2(name='a2_1') a2_2 = A2(name='a2_2') a2_3 = A2(name='a2_1') a1 = A1() for thing in (a2_1, a2_2, a2_3, a1): _session.add(thing) for a in (a2_1, a2_2, a2_3, a2_1): a1.a2.append(a) _session.flush() assert len(a1.a2) == 2

当然，必须根据需要实施进一步的行为，例如__getitem__() / __setitem__() / __delitem__()用于编制索引等。

SQLAlchemy：如何通过事件监听器有条件地添加到集合中？

1 个答案: