Add a hook to run queries between the start of a transaction and a view

[Sven R. Kunze]

We are upgrading from Django 1.7 to Django 1.8. We noticed, we cannot perform things like:

connection.cursor().execute('SET LOCAL my_variable TO %d' % var)

in some of our Middlewares anymore due to the missing TransactionMiddleware.

We have more than 2,000 views (some of them created dynamically) , so decorating them manually is not a practical nor secure solution.

Currently, we patch BaseHandler.make_view_atomic to handle this but its weird that there is no default way to insert custom code in between starting a transaction and the actual execution of a view.

[Simon Charette]

Hi srkunze, ​TransactionMiddleware has been deprecated ​since Django 1.6 in favor of the ​ATOMIC_REQUESTS settings which should do exactly what you are expecting.

Please reopen if adding ATOMIC_REQUESTS: True to your DATABASES setting aliases doesn't work for your.

[Aymeric Augustin]

I believe the transaction created by ATOMIC_REQUESTS only applies to the view, not to middleware.

As a consequence it may not solve the reporter's problem (but I'm not sure I understood it entirely).

[Sven R. Kunze]

I believe the transaction created by ATOMIC_REQUESTS only applies to the view, not to middleware.

Exactly. That's why we monkey patched ​BaseHandler.make_view_atomic.

Basically like this:

def make_view_atomic(self, view):
    non_atomic_requests = getattr(view, '_non_atomic_requests', set())
    for db in connections.all():
        if (db.settings_dict['ATOMIC_REQUESTS']
                and db.alias not in non_atomic_requests):
            view = transaction.atomic(using=db.alias)(yet_another_wrapper(view))
    return view

def yet_another_wrapper(view):
    def wrapped_view(request, *args, **kwargs):
        # do what legacy systems usually do
        # weird stuff
        # user-and-transaction-related code that always needs to run
        # custom things

        # actual view
        return view(request, *args, **kwargs)
    return wrapped_view

[Sven R. Kunze]

@timgraham I changed the code example to pass db into the wrapper function.

Please, note we would need to execute arbitrary code (not just queries) between start of a transaction and all views.

[Thomas Güttler]

Just for the records. The same topic was discussed in February 2015: ​https://groups.google.com/forum/#!topic/django-developers/uW5Ogio8QBc

[Aymeric Augustin]

At that point I would suggest to write your own transaction middleware that does:

self.atomic = transaction.atomic()
self.atomic.__enter__()

in the request phase and

self.atomic.__exit__(...)

in the response phase.

Django has gotten away with roughly the equivalent of this until version 1.6.

Sorry, I'm not proud of that...

[Sven R. Kunze]

@aaugustin We'd like to use the ATOMIC_REQUESTS feature unbroken as is.

A simple hook, as timgraham suggested, would fit the bill.

[Aymeric Augustin]

But that would increase complexity to an area that is already poorly designed (by my fault), thus making it even harder to refactor in the future :-/

[Tim Graham]

If I understand correctly, the ​new middleware proposal would allow implementing a TransactionMiddleware with the same functionality as ATOMIC_REQUESTS with the benefit that you could also have other middleware to run queries like requested here, so that would solve this ticket.

[Florian Apolloner]

@tim: Yes that was the main motivation of the middleware refactor. We should have TransactionMiddleware in 1.10 again.

[Thomas Güttler]

The new "DEP 0005: Re-thinking middleware" would solve this issue:

..... It also allows more powerful idioms that aren't currently possible, like wrapping the call to get_response in a context manager (e.g. transaction.atomic) or in a try/finally block.

Source: ​https://github.com/django/deps/blob/master/draft/0005-improved-middleware.rst

A big thank you to Carl Meyer, the author of DEP 0005.

[Tim Graham]

Closing in light of the middleware refactor.