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Jul 1, 2010

Overriding QuerySet in Django

Vladimir Sidorenko

As you know, model managers can be overriden in Django. It's convenient to add custom filtration method there:

Article.objects.published()
    Article.objects.old()

But these custom methods cannot be chained:

Article.objects.published().old()

Overriding manager doesn't allow it, because after first manager method is invoked (publishedin this case), we receive queryset as a result, which knows nothing about custom manager methods. So we have to add custom methods to queryset. This can look like this:

class ArticleQuerySet(models.query.QuerySet):

        def published(self):
            return self.filter("...")

        def old(self):
            return self.filter("...")

Now we need to make manager user this class. In order to do this, let's override manager's get_query_set method:

class ArticleManager(models.Manager):

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

    class Article(models.Model):

        # ...

        objects = ArticleManager()

This allows to make queties like:

Article.objects.all().published().old()

Here we invoke all to get queryset instance, because this time manager doesn't have custom methods - publishedand old. To avoid this, we can make manager look for undefined methods in queryset:

class ArticleManager(models.Manager):

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

        def getattr(self, key):
            return getattr(self.get_query_set(), key)

Note that getattris only invoked when attribute is not found by usual means.

Manager can be further improved to be more generic:

class QuerySetManager(models.Manager):

        def init(self, queryset_class, args, **kwargs):
            self.queryset_class = queryset_class
            super(QuerySetManager, self).init(args, **kwargs)

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

        def getattr(self, key):
            return getattr(self.get_query_set(), key)

       # Теперь можно писать просто:
       # objects = QuerySetManager(ArticleQuerySet)
       #

Update

After article was published, I found out that this idea is not all that new and unique :)

It's funny that only after I implemented my own solution, I've managed to make correct search query.

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