提交某种类型的模型后运行函数

我想在实例时运行一个函数Post模型已承诺。我想运行它any他们提交的时间，所以我不想在任何地方显式调用该函数。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.

无论您选择下面的哪个选项，SQLAlchemy 都会附带关于的一个重大警告after_commit event http://docs.sqlalchemy.org/en/latest/orm/events.html#sqlalchemy.orm.events.SessionEvents.after_commit（这是两种方式都发送信号的时候）。

当会话不处于活动事务中时after_commit()事件被调用，因此无法发出 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，您可以使用任务队列，例如Celery http://www.celeryproject.org/在后台线程中执行此操作（使用单独的会话）。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，并且您不希望视图在发送电子邮件时等待返回。

Flask-SQLAlchemy提供信号 http://flask-sqlalchemy.pocoo.org/dev/signals/#models_committed您可以收听发送所有插入/更新/删除操作的声音。需要通过设置来启用app.config["SQLALCHEMY_TRACK_MODIFICATIONS"] = True因为跟踪修改的成本很高，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， 不开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)