Back
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:

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 (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:

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

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)

 

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.

Subscribe for the news and updates

More thoughts
Jul 21, 2022Technology
Codemirror: unit-testing codemirror react components

One of our recent projects includes the functionality of an inline code editor. This code editor needed to be highly extensible and have custom features. To address this, we chose Codemirror v6 due to its peculiar architecture - it is highly customizable, and all the additional features are provided into codemirror engine as Extension objects.

Mar 2, 2017Technology
API versioning with django rest framework?

We often handling API server updates including backwards-incompatible changes when upgrading web applications. At the same time we update the client part, therefore, we did not experience any particular difficulties.

Feb 28, 2017Technology
How to write an API in Django

There is such a term as Remote Procedure Call (RPC). In other words, by using this technology, programs can call functions on remote computers. There are many ways to implement RPC.

Jan 28, 2017Technology
Creating a site preview like in slack (using aiohttp)

In this article we will write a small library for pulling metadata and creating a preview for a site just like Slack does.

Oct 22, 2016Technology
Solr Sharding

When dealing with one of our projects (LookSMI media monitoring platform) we have to handle the huge volume of data – and its quantity is constantly growing. At the same time, we must run quick searches with smart rules. In this article I'll explain how we have achieved required performance.

May 12, 2010Technology
Twitter API, OAuth and decorators

In my current project I had a task to use twitter API. Twitter uses OAuth for authentication, which is pretty dreary. To avoid fiddling with it all the time, I've moved authentication to decorator. If key is available - nothing happens, just view is launched as usual. It's convenient that there's no need for additional twitter settings in user profile. Code is in article.