Jul 1, 2010

Overriding QuerySet in Django

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


But these custom methods cannot be chained:


Overriding manager doesn't allow it, because after first manager method is invoked (published in 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:


Here we invoke all to get queryset instance, because this time manager doesn't have custom methods - published and 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 getattr is 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)

# Now you can simply write:
# objects = QuerySetManager(ArticleQuerySet)



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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