Django Auto-Prefetching

Never worry about n+1 performance problems again

This project aims to automatically perform the correct select_related and prefetch_related calls for your django-rest-framework code. It does this by inspecting your serializers, seeing what fields they use, and what models they refer to, and automatically calculating what needs to be prefetched.

Installation

Installation via pip:

pip install django-auto-prefetching

AutoPrefetchViewSetMixin

This is a ViewSet mixin you can use, which will automatically prefetch the needed objects from the database, based on the ViewSets queryset and serializer_class. Under most circumstances this will be all the database optimizations you'll ever need to do:

Usage

Simply add it after your ModelViewSet class.

from django_auto_prefetching import AutoPrefetchViewSetMixin
from rest_framework.viewsets import ModelViewSet

class BaseModelViewSet(AutoPrefetchViewSetMixin, ModelViewSet):
    queryset = YourModel.objects.all()
    serializer_class = YourModelSerializer

It supports all types of relational fields (many to many, one to many, one to one etc.) out of the box.

Manually calling prefetch

The AutoPrefetchViewSetMixin cannot see what objects are being accessed in e.g. a SerializerMethodField. If you use objects in there, you might need to do some additional prefetches. If you do this and override get_queryset, you will have to call prefetch manually as the mixin code is never reached.

import django_auto_prefetching
from rest_framework.viewsets import ModelViewSet

class BaseModelViewSet(django_auto_prefetching.AutoPrefetchViewSetMixin, ModelViewSet):
    serializer_class = YourModelSerializer
    
    def get_queryset(self):
        # Simply do the extra select_related / prefetch_related here
        # and leave the mixin to do the rest of the work
        queryset = YourModel.objects.all()
        queryset = queryset.select_related('my_extra_field')
        return django_auto_prefetching.prefetch(queryset, self.serializer_class)

You can override get_prefetchable_queryset instead of get_queryset if you don't want to manually call django_auto_prefetching.prefetch(). Example:

import django_auto_prefetching
from rest_framework.viewsets import ModelViewSet

class BaseModelViewSet(django_auto_prefetching.AutoPrefetchViewSetMixin, ModelViewSet):
    serializer_class = YourModelSerializer
    
    def get_prefetchable_queryset(self):
        return YourModel.objects.all()

Now get_queryset() will call our get_prefetchable_queryset() and will add automatic prefetches

Manually specifying which fields are needed

If you need to explicitly specify some extra fields to be included or excluded, you can also override the following methods on your ViewSet to return a list or a set of fields to prefetch/exclude.

import django_auto_prefetching
from rest_framework.viewsets import ModelViewSet

class BaseModelViewSet(django_auto_prefetching.AutoPrefetchViewSetMixin, ModelViewSet):
    serializer_class = YourModelSerializer

    def get_auto_prefetch_excluded_fields(self):
        return {"exclude_this_field", "and_this_field"}
    
    def get_auto_prefetch_extra_select_fields(self):
        return {"select_related_on_this_field"}

    def get_auto_prefetch_extra_prefetch_fields(self):
        return {"prefetch_related_on_this_field"}
}