在提交某种类型的模型后运行函数

Question

我想Post在提交模型实例时运行一个函数.我想在它们提交的任何时候运行它,所以我宁愿不在任何地方明确地调用该函数.我怎样才能做到这一点？

def notify_subscribers(post):
    """ send email to subscribers """
    ...

post = Post("Hello World", "This is my first blog entry.")
session.commit() # How to run notify_subscribers with post as argument
                 # as soon as post is committed successfully?

post.title = "Hello World!!1"
session.commit() # Run notify_subscribers once again.

Answer 1

无论您在下面选择哪个选项，SQLAlchemy 都会发出有关该after_commit事件的重大警告（即双向发送信号的时间）。

当after_commit()事件被调用时 Session 不在活动事务中，因此不能发出 SQL。

如果您的回调需要查询或提交到数据库，则可能会出现意外问题。在这种情况下，您可以使用诸如Celery 之类的任务队列在后台线程（使用单独的会话）中执行此操作。无论如何，这可能是正确的方法，因为发送电子邮件需要很长时间，而且您不希望视图在发生时等待返回。

Flask-SQLAlchemy提供了一个您可以收听的信号，该信号发送所有插入/更新/删除操作。它需要通过设置来启用，app.config["SQLALCHEMY_TRACK_MODIFICATIONS"] = True因为跟踪修改很昂贵并且在大多数情况下不需要。

然后监听信号：

from flask_sqlalchemy import models_committed

def notify_subscribers(app, changes):
    new_posts = [target for target, op in changes if isinstance(target, Post) and op in ('insert', 'update')]
    # notify about the new and updated posts

models_committed.connect(notify_subscribers, app)

app.config["SQLALCHEMY_TRACK_MODIFICATIONS"] = True

您也可以自己实现（主要是通过从 Flask-SQLAlchemy 复制代码）。这有点棘手，因为模型更改发生在flush，而不是commit，因此您需要在刷新发生时记录所有更改，然后在提交后使用它们。

from sqlalchemy import event

class ModelChangeEvent(object):
    def __init__(self, session, *callbacks):
        self.model_changes = {}
        self.callbacks = callbacks

        event.listen(session, 'before_flush', self.record_ops)
        event.listen(session, 'before_commit', self.record_ops)
        event.listen(session, 'after_commit', self.after_commit)
        event.listen(session, 'after_rollback', self.after_rollback)

    def record_ops(self, session, flush_context=None, instances=None):
        for targets, operation in ((session.new, 'insert'), (session.dirty, 'update'), (session.deleted, 'delete')):
            for target in targets:
                state = inspect(target)
                key = state.identity_key if state.has_identity else id(target)
                self.model_changes[key] = (target, operation)

    def after_commit(self, session):
        if self._model_changes:
            changes = list(self.model_changes.values())

            for callback in self.callbacks:
                callback(changes=changes)

            self.model_changes.clear()

    def after_rollback(self, session):
        self.model_changes.clear()

def notify_subscribers(changes):
    new_posts = [target for target, op in changes if isinstance(target, Post) and op in ('insert', 'update')]
    # notify about new and updated posts

# pass all the callbacks (if you have more than notify_subscribers)
mce = ModelChangeEvent(db.session, notify_subscribers)
# or you can append more callbacks
mce.callbacks.append(my_other_callback)