Custom QuerySet and Manager without breaking DRY?

Question

I'm trying to find a way to implement both a custom QuerySet and a custom Manager without breaking DRY. This is what I have so far:

class MyInquiryManager(models.Manager):
    def for_user(self, user):
        return self.get_query_set().filter(
                    Q(assigned_to_user=user) |
                    Q(assigned_to_group__in=user.groups.all())
                )

class Inquiry(models.Model):   
    ts = models.DateTimeField(auto_now_add=True)
    status = models.ForeignKey(InquiryStatus)
    assigned_to_user = models.ForeignKey(User, blank=True, null=True)
    assigned_to_group = models.ForeignKey(Group, blank=True, null=True)
    objects = MyInquiryManager()

This works fine, until I do something like this:

inquiries = Inquiry.objects.filter(status=some_status)
my_inquiry_count = inquiries.for_user(request.user).count()

This promptly breaks everything because the QuerySet doesn't have the same methods as the Manager. I've tried creating a custom QuerySet class, and implementing it in MyInquiryManager, but I end up replicating all of my method definitions.

I also found this snippet which works, but I need to pass in the extra argument to for_user so it breaks down because it relies heavily on redefining get_query_set.

Is there a way to do this without redefining all of my methods in both the QuerySet and the Manager subclasses?

Solution 1

Django has changed! Before using the code in this answer, which was written in 2009, be sure to check out the rest of the answers and the Django documentation to see if there is a more appropriate solution.

---

The way I've implemented this is by adding the actual get_active_for_account as a method of a custom QuerySet. Then, to make it work off the manager, you can simply trap the __getattr__ and return it accordingly

To make this pattern re-usable, I've extracted out the Manager bits to a separate model manager:

custom_queryset/models.py

from django.db import models
from django.db.models.query import QuerySet

class CustomQuerySetManager(models.Manager):
    """A re-usable Manager to access a custom QuerySet"""
    def __getattr__(self, attr, *args):
        try:
            return getattr(self.__class__, attr, *args)
        except AttributeError:
            # don't delegate internal methods to the queryset
            if attr.startswith('__') and attr.endswith('__'):
                raise
            return getattr(self.get_query_set(), attr, *args)

    def get_query_set(self):
        return self.model.QuerySet(self.model, using=self._db)

Once you've got that, on your models all you need to do is define a QuerySet as a custom inner class and set the manager to your custom manager:

your_app/models.py

from custom_queryset.models import CustomQuerySetManager
from django.db.models.query import QuerySet

class Inquiry(models.Model):
    objects = CustomQuerySetManager()

    class QuerySet(QuerySet):
        def active_for_account(self, account, *args, **kwargs):
            return self.filter(account=account, deleted=False, *args, **kwargs)

With this pattern, any of these will work:

>>> Inquiry.objects.active_for_account(user)
>>> Inquiry.objects.all().active_for_account(user)
>>> Inquiry.objects.filter(first_name='John').active_for_account(user)

UPD if you are using it with custom user(AbstractUser), you need to change
from

class CustomQuerySetManager(models.Manager):

to

from django.contrib.auth.models import UserManager

class CustomQuerySetManager(UserManager):
    ***

Solution 2

The Django 1.7 released a new and simple way to create combined queryset and model manager:

class InquiryQuerySet(models.QuerySet):
    def for_user(self, user):
        return self.filter(
            Q(assigned_to_user=user) |
            Q(assigned_to_group__in=user.groups.all())
        )

class Inquiry(models.Model):
    objects = InqueryQuerySet.as_manager()

See Creating Manager with QuerySet methods for more details.

Solution 3

You can provide the methods on the manager and queryset using a mixin.

This also avoids the use of a __getattr__() approach.

from django.db.models.query import QuerySet

class PostMixin(object):
    def by_author(self, user):
        return self.filter(user=user)

    def published(self):
        return self.filter(published__lte=datetime.now())

class PostQuerySet(QuerySet, PostMixin):
    pass

class PostManager(models.Manager, PostMixin):
    def get_query_set(self):
        return PostQuerySet(self.model, using=self._db)

Solution 4

You can now use the from_queryset() method on you manager to change its base Queryset.

This allows you to define your Queryset methods and your manager methods only once

from the docs

For advanced usage you might want both a custom Manager and a custom QuerySet. You can do that by calling Manager.from_queryset() which returns a subclass of your base Manager with a copy of the custom QuerySet methods:

class InqueryQueryset(models.Queryset):
    def custom_method(self):
        """ available on all default querysets"""

class BaseMyInquiryManager(models.Manager):
    def for_user(self, user):
        return self.get_query_set().filter(
                    Q(assigned_to_user=user) |
                    Q(assigned_to_group__in=user.groups.all())
                )

MyInquiryManager = BaseInquiryManager.from_queryset(InquiryQueryset)

class Inquiry(models.Model):   
    ts = models.DateTimeField(auto_now_add=True)
    status = models.ForeignKey(InquiryStatus)
    assigned_to_user = models.ForeignKey(User, blank=True, null=True)
    assigned_to_group = models.ForeignKey(Group, blank=True, null=True)
    objects = MyInquiryManager()

Solution 5

A slightly improved version of T. Stone’s approach:

def objects_extra(mixin_class):
    class MixinManager(models.Manager, mixin_class):
        class MixinQuerySet(QuerySet, mixin_class):
            pass

        def get_query_set(self):
            return self.MixinQuerySet(self.model, using=self._db)

    return MixinManager()

Class decorators make usage as simple as:

class SomeModel(models.Model):
    ...
    @objects_extra
    class objects:
        def filter_by_something_complex(self, whatever parameters):
            return self.extra(...)
        ...

Update: support for nonstandard Manager and QuerySet base classes, e. g. @objects_extra(django.contrib.gis.db.models.GeoManager, django.contrib.gis.db.models.query.GeoQuerySet):

def objects_extra(Manager=django.db.models.Manager, QuerySet=django.db.models.query.QuerySet):
    def oe_inner(Mixin, Manager=django.db.models.Manager, QuerySet=django.db.models.query.QuerySet):
        class MixinManager(Manager, Mixin):
            class MixinQuerySet(QuerySet, Mixin):
                pass

            def get_query_set(self):
                return self.MixinQuerySet(self.model, using=self._db)

        return MixinManager()

    if issubclass(Manager, django.db.models.Manager):
        return lambda Mixin: oe_inner(Mixin, Manager, QuerySet)
    else:
        return oe_inner(Mixin=Manager)

View more solutions

Comments

[Jack M.]

Try doing a filter, then using get_active_for_account. It works in your example, but not once you've already used a filter, and are then working with a QuerySet, which was my example.

[Sam Saffron]

can you decide what should be done with stackoverflow.com/edit-suggestions/1216

[Sam Saffron]

Stone you are going to have to edit it in yourself, there is no way to take an edit after it was declined

[Aneil Mallavarapu]

WARNING: I tried this method and discovered that it severely slows down .defer and .only calls.

[T. Stone]

I've updated this response to a community wiki. Those with performance optimizations can adjust the code as necessary.

[Jelko]

probably similar to PassThroughManager from pypi.python.org/pypi/django-model-utils

[Agustín Lado]

This is the best way to do it, but it'd be got to exemplify how the for_user method should take a user and return self.[...] to chain together multiple operations.

[Vadim Pushtaev]

That's pretty awesome, they have such decorator in the django itself

[Vadim Pushtaev]

My Django wants get_queryset to be overriden, not get_query_set.

[Anentropic]

Folks, I found a nasty bug in the implementation above, I have edited it with the fix. For explanation of the problem see here gist.github.com/anentropic/0f2d700b5abdc21177bb This must be what @AneilMallavarapu meant when saying it slowed down defer and only calls... due to pathological extra queries being performed. Fixed now with current edit.

[Barney Szabolcs]

Funny how this is not he first answer here. Professionally this is the cleanest solution.

[Sian Lerk Lau]

The solution provided by @Anentropic is working, though we can improvise it by honoring it's original behavior as per python data model. The outcome will be as follow: gist.github.com/kiawin/13e56e47bde59d9d02112c3d9c373b0d. tl;dr - __getattr__ only accepts one parameter (we don't need *args), and it is triggered only if the instance attribute is not found (we don't need to do a try-catch)

[shahjapan]

I like your solution ! as all methods defined in custom-queryset are available to manager. :-)