Fixed #23533 -- Allowed defining initial filters on QuerySet classes.

[felixxm]

Thanks Simon Charette for the implementation idea.

ticket-23533

[charettes]

Still short on time today but I could swear I had a branch somewhere to approach this problem slightly differently.

By defining a class_or_instance_method like object that accumulates method calls when called on a class in a generic way you could extend this pattern to any QuerySet method easily. e.g.

class queryset_method:
    def __init__(self, method):
        self.method = method

    def buffer_call(self, cls, *args, **kwargs):
        cls._buffered_calls.append((self.method.__name__, args, kwargs))
        return cls
 
    def __get__(self, instance, owner):
        if instance is None:
            # Avoid creating intemediary types for chained calls.
            if (buffered_calls := getattr(owner, "_buffered_calls", None)) is None:
                cls = type(
                    f"Buffered{owner.__name__}",
                    (owner,),
                    {"_buffered_calls": []},
                )
            else:
                cls = owner
            return partial(self.buffer_call, cls)
        return self.method

then at queryset construction time you simply have to iterate on ._buffered_calls and apply all the method calls. Assuming you decorate filter, exclude, select_related and friend you'd be able to do

class Book(models.Model):
    ...
    published_objects = models.QuerySet.exclude(
        published_at=None,
    ).select_related("author").as_manager()

You do loose the level of immediate validation you get in your specialized class method (e.g. PROHIBITED_FILTER_KWARGS) but you still get it at queryring time just like you would get if you wrote your owner Manager.get_queryset subclass and passed invalid kwargs.

[charettes]

We'll likely need a way to pickle these dynamically objects.