How to avoid n+1 select in django?

Question

I have a very simple datamodel with a one to many relationship between video and comments

class Video(models.Model):
    url = models.URLField(unique=True)
    .....

class Comment(models.Model):
    title = models.CharField(max_length=128)
    video = models.ForeignKey('Video')
        .....

I want to query for videos and grab the whole object graph (videos with all the comments). Looking at the sql, I see it does two selects, one for the Videos and one for the Comments. How do I avoid that? I want to do a join and grab everything at once.

Is it possible to do this with django?

Answer 1

For ForeignKey, you can use selected_related():

Comment.objects.select_related('video').all()

It will generate one query only, gathering coments for you as well as videos.

For something more complex (such as M2M), you need an external app such as unjoinify to make optimizations but it uses SQL queries to then put them back in objects.

If you are unconfortable with this (I am), you have some alternatives:

• django-queryset-transform: not a full solution, but helps
• django-batch-select: roughtly a select_related that works with M2M and reverse relations.