Django, multiple DB hits when repeating one related query

Question

This is the model:

class Category(models.Model):
    name = models.TextField()

class Post(models.Model):
    category = models.ForeignKey(Category)

Now, I want to get posts of a category:

category = Category.objects.get(id=1)

posts = category.post_set.all()
# this line hit the DB

posts = category.post_set.all()
# and this line hit the DB again!

How to use the cached result in these relations. I use Django rest-framework and it makes DB hit multiple times for each instance.

willeM_ Van Onsem · Accepted Answer · 2021-08-22 12:15:07Z

2

You can work with .prefetch_related(…) [Django-doc]:

category = Category.objects.prefetch_related('posts').get(id=1)

This will load the related objects, and will do the JOINing at the Django/Python layer. This thus means that the .all() calls will use the prefetched objects.

answered Aug 22, 2021 at 12:15

willeM_ Van Onsem

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1 Comment

mrash Over a year ago

I using category = Category.objects.prefetch_related("post_set") in drf view get_queryset function and its worked!

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