Django snippets: Custom managers with chainable filters

Custom managers with chainable filters

from django.db import models

class NewsQuerySet(models.query.QuerySet):
    def live(self):
        return self.filter(state='published')

    def interesting(self):
        return self.filter(interesting=True)

class NewsManager(models.Manager):
    def get_query_set(self): 
        model = models.get_model('news', 'NewsItem')
        return NewsQuerySet(model)

    def __getattr__(self, attr, *args):
        try:
            return getattr(self.__class__, attr, *args)
        except AttributeError:
            return getattr(self.get_query_set(), attr, *args)

The Django docs show us how to give models a custom manager. Unfortunately, filter methods defined this way cannot be chained to each other or to standard queryset filters. Try it:

class NewsManager(models.Manager):
    def live(self):
        return self.filter(state='published')

    def interesting(self):
        return self.filter(interesting=True)

>>> NewsManager().live().interesting()
AttributeError: '_QuerySet' object has no attribute 'interesting'

So, instead of adding our new filters to the custom manager, we add them to a custom queryset. But we still want to be able to access them as methods of the manager. We could add stub methods on the manager for each new filter, calling the corresponding method on the queryset - but that would be a blatant DRY violation. A custom __getattr__ method on the manager takes care of that problem.

And now we can do:

>>> NewsManager().live().interesting()
[<NewsItem: ...>]

Author:: itavor
Posted:: January 23, 2008
Language:: Python
Tags:: manager queryset
Score:: 7 (after 7 ratings)

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ep (on January 26, 2008):

Thanks, looks excellent!

I think you can even avoid repeating the Manager code for each of your classes by using something like:

class QuerySetManager(models.Manager):
    def __init__(self, qs_class):
        self.queryset_class = qs_class
    def get_query_set(self):
        return self.queryset_class(self.model)
    def __getattr__(self, attr, *args):
        try:
            return getattr(self.__class__, attr, *args)
        except AttributeError:
            return getattr(self.get_query_set(), attr, *args)

and then setting the objects attribute like this:

class NewsItem(models.Model):
    objects = QuerySetManager(NewsQuerySet)

itavor (on February 1, 2008):

Thanks, ep! I like your improvement. Will do it that way in my own code from now on.

herion (on April 20, 2008):

I tried using this. In ep's approach Manager's init is overriden and it expects an additional argument compared to Django core's Manager. However, in Django code Manager's are called without arguments. So when using object.delete() and object.someothermodel_set.* methods the system tries to call the Manager without arguments and it throws an error. I don't know however, why Django is calling Manager via the Manager model and not thourgh models Manager instance (so it doesn't help if you make argument optional as class can't find the queryset then).. I found ep's approach much too hazard although you could hack it here and there..

whiteinge (on July 13, 2008):

herion, just use a default for the qs_class:

def __init__(self, qs_class=models.query.QuerySet):
    self.queryset_class = qs_class

Django snippets

Custom managers with chainable filters

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