Question

假设我有类Dog（），Walrus（），Boot（）。我想做它，所以你不能更新Walrus对象，虽然你可以删除它们，你永远不能删除Boot对象。所以如果：

dog1 = Dog("DogName")
walrus1 = Walrus("WalrusName")
boot1 = Boot("BootName")
session.add(dog1)
session.add(walrus1)
session.add(boot1)
session.flush()
transaction.commit()
dog1.name = "Fluffy"
walrus1.name = "Josh"
boot1.name = "Pogo"
session.flush()
transaction.commit()

它会在更改海象名称时引发异常，但允许更改其他名称。如果我试图删除boot1，它会抛出异常。

我对事件听众采取了一些刺激，但我接近的两种方式并没有让我一直到那里：

#One possibility
#I don't know how to tell that it's just an update though
#The is_modified seems to take inserts as well

@event.listens_for(Session, 'before_flush')
def listener(thissession, flush_context, instances):
    for obj in thissession:
        if isinstance(obj, Walrus):
            if thissession.is_modified(obj, include_collections=False):
                thissession.expunge(obj)


#Possiblity two
#It says before_update but it seems to take in inserts as well
#Also documentation says it's not completely reliable to capture all statements
#where an update will occur
@event.listens_for(Walrus, 'before_update', raw=True)
def pleasework(mapper, connection, target):
    print "\n\nInstance %s being updated\n\n" % target
    object_session(target).expunge(target)

编辑1：

@event.listens_for(Walrus, 'before_update', raw=True)
def prevent_walrus_update(mapper, connection, target):
    print "\n\nInstance %s being updated\n\n" % target
    if target not None:
        raise

@event.listens_for(Boot, 'before_delete', raw=True)
def prevent_boot_delete(mapper, connection, target):
    print "\n\nInstance %s being deleted\n\n" % target
    if target not None:
        raise

我已经让它工作了，它不允许我对Walrus进行更新或删除Boot，但任何暗示试图将崩溃的AttributeError我似乎没有任何能力抓住。例如，如果我运行Walrus1.name =“Josh”然后在al处进行任何查询，即使是get，AttributeError也会使应用程序崩溃。我比我更进一步，但仍然很不方便。

Answer 1

如果您只是将循环更改为：

，那么您发布的第一个解决方案看起来应该可行

for obj in thissession.dirty:

您可以使用相同的before_flush事件来阻止通过thissession.deleted循环删除Boot对象。

如何防止在SQLAlchemy中更新/删除特定的类？

1 个答案: