Documentation on creating custom querysets

[lee@…]

Currently Django's documentation describes how customize the manager's initial query set (​https://docs.djangoproject.com/en/dev/topics/db/managers/#modifying-initial-manager-querysets), but not how to create filters that can be used on an existing query set, at any position in the chain of filters.

Doing so requires subclassing the queryset object. This blog post describes the issue in more detail and proposes a solution:

​http://zmsmith.com/2010/04/using-custom-django-querysets/

It would be helpful if there were an officially approved way to subclass querysets or add custom chainable filters, and it was part of the documentation.

[Aymeric Augustin]

The solution described in the blog post requires five lines of boilerplate code, which is a blocker for recommending it in the official documentation:

class CustomManager(models.Manager):

    def get_query_set(self):
        return CustomQuerySet(self.model)

    def __getattr__(self, name):
        return getattr(self.get_query_set(), name)

Currently, the docs just describe the APIs of Manager and QuerySet.

I don't know if there's a general solution to this problem, if we want to develop one in Django, or just let each developer figure out what's most appropriate in his/her situation.

[Jacob]

There really ought to be a better way of making a manager and custom queryset all in one step -- it's a common operation. But until then we should document the crufty way.

[anonymous]

How about something like this:
​https://github.com/zacharyvoase/django-qmethod

[Mike Fogel]

Note that the __getattr__ definition in the above boilerplate code violates ​https://docs.djangoproject.com/en/dev/topics/db/managers/#implementation-concerns

Associated discussion on #15062.

[alan@…]

Replying to anonymous:

How about something like this:
​https://github.com/zacharyvoase/django-qmethod

I second this method (or at least its syntax). I haven't tested it yet in my project, but it looks quite elegant. To me, the use of a separate Manager class makes sense when you might reuse that manager for two different models, but for my app, I usually just need one-off managers anyway. Being able to define custom queryset methods inline in the model class in most cases would make my business logic much simpler.

[jeremyt]

Would something like adding queryset class as an optional parameter to models.Manager make the story a bit cleaner?

class CustomManager(models.Manager):
    def __init__(self, query_set_class=CustomQuerySet):
        super(CustomManager, self).__init__(query_set_class=query_set_class)

The code change would look like this: ​https://github.com/jtillman/django/compare/CustomQuerySet

[loic84]

Newer ticket with related discussion: #20625

[Tim Graham]

It seems like the related ticket has momentum, so it's probably not worth documenting "the crufty way" at this point.