Warning in documentation about filtering queryset in ClassBasedViews

[Daniel Gayoso González]

Hello,

Following the example in https://docs.djangoproject.com/en/4.1/topics/class-based-views/generic-display/#viewing-subsets-of-objects I tried the following

class BookListView(ListView):

    model = Book

    queryset = Book.objects.filter(publication_date__lte=timezone.now())

I found that this code snippet to retrieve only books that has the publication date before today (assuming that could be books with publication date in the future) not work as I would expected. 

After some digging, I found that timezone.now() it's cached when server starts up (in a production environment), so this query filter by the date the server was started. So the solution is to use a dynamic filtering. 

Could be a good idea to include some warning about this in the documentation?

Thanks,

Daniel

[David Sanders]

Hi Daniel,

I don't think a warning is necessary here as it's fairly standard Python.

To explain: the timezone.now() is evaluated at module level – ie only once when the Python module is imported – which explains the behaviour that you're experiencing.

If you like you can submit a documentation PR with some suggestions but others may also doubt the value of adding a warning given that the very next section explains how to do dynamic filtering. Personally I think by the time you get to advanced Django such as this, Python experience is assumed :)

Regards,

David

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[Adam Johnson]

I'm inclined to agree with David here. This is a “understanding Python” thing. We can’t feasibly add documentation warnings everywhere such a mistake could be made.

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[danig...@gmail.com]

I thought that it would a good idea to point this because it is a bug that you will not catch until production stage, but I guess time dependant queries are prone to this and require experience.

After reading your comments I realize this is more a python problem than a Django one, as both of you said.

Thank you for your feedback on this