Quick query optimization in bare Django project

You’ve just installed Django and started new project without any models or anything else. Created superuser and jumped in to User Admin… What will you see ? 35 queries…

Why ? Permission model’s ForeignKey relation with ContentType model mostly causes this performance issue. If you check the source code, you’ll see the problem;

class Permission(models.Model):
    :
    :
    def __str__(self):
        return "%s | %s | %s" % (
            self.content_type.app_label,    # <- this
            self.content_type,              # <- this
            self.name,
        )

Well, let’s not call this a problem… We are lucky because Django provides great solutions. Before we fix this, let’s verify that two fields are making this big queries: user_permissions and groups in User model. How I verified ? Just excluded those two fields from Admin Form:

# for the sake of example, there is no app installed, no models, nothing.
# therefore i'm using `urls.py` for demonstration
# urls.py

from django.contrib import admin
from django.contrib.auth.models import User
from django.urls import path

class UserAdmin(admin.ModelAdmin):
    list_filter = ['is_staff']
    list_display = ('email', 'first_name', 'last_name')
    exclude = ('groups', 'user_permissions')

admin.site.unregister(User)            # get rid of Django's default UserAdmin
admin.site.register(User, UserAdmin)   # register our UserAdmin

Woow! We have generated 5 queries only… From 35 to 5 by excluding two fields…

Now the best is coming… After Django version 2, ModelAdmin gained a new feature/option called autocomplete_fields. Let’s transform previous code:

from django.conf import settings
from django.contrib import admin
from django.urls import include, path
from django.contrib.auth.models import User, Permission

class PermissionAdmin(admin.ModelAdmin):
    search_fields = ['name']

class UserAdmin(admin.ModelAdmin):
    list_filter = ['is_staff']
    list_display = ('email', 'first_name', 'last_name')
    autocomplete_fields = ['groups', 'user_permissions']

admin.site.unregister(User)
admin.site.register(User, UserAdmin)
admin.site.register(Permission, PermissionAdmin)

Now we have generated 7 queries only! If you hit refresh, Django will use cached query and you’ll see 6 queries only :)

You can also override ModelAdmin’s formfield_for_manytomany and make select_related and prefetch_related queries on kwargs['queryset'] or you can modify UserAdminForm on __init__ level too. ModelAdmin’s autocomplete_fields option is a query and life saver!

Again, do not use urls.py for adding custom Admin. Use your model’s Admin admin.py. I mostly use custom User model in my projects therefore I put related code there.