django-zeal

django-zeal

Catch N+1 queries in your Django project.

🔥 Battle-tested at Cinder

Features

• Detects N+1s from missing prefetches and from use of .defer()/.only()
• Friendly error messages like N+1 detected on social.User.followers at myapp/views.py:25 in get_user
• Configurable thresholds
• Allow-list
• Well-tested
• No dependencies

Acknowledgements

This library draws heavily from jmcarp's nplusone. It's not a fork, but a lot of the central concepts and initial code came from nplusone.

Installation

First:

pip install django-zeal

Then, add zeal to your INSTALLED_APPS and MIDDLEWARE.

if DEBUG:
    INSTALLED_APPS.append("zeal")
    MIDDLEWARE.append("zeal.middleware.zeal_middleware")

This will detect N+1s that happen in web requests. To catch N+1s in more places, read on!

[!WARNING]
You probably don't want to run zeal in production: there is significant overhead to detecting N+1s, and my benchmarks show that it can make your code between 2.5x - 7x slower.

Celery

If you use Celery, you can configure this using signals:

from celery.signals import task_prerun, task_postrun
from zeal import setup, teardown
from django.conf import settings

@task_prerun.connect()
def setup_zeal(*args, **kwargs):
    setup()

@task_postrun.connect()
def teardown_zeal(*args, **kwargs):
    teardown()

Tests

Django runs tests with DEBUG=False, so to run zeal in your tests, you'll first need to ensure it's added to your INSTALLED_APPS and MIDDLEWARE. You could do something like:

import sys

TEST = "test" in sys.argv
if DEBUG or TEST:
    INSTALLED_APPS.append("zeal")
    MIDDLEWARE.append("zeal.middleware.zeal_middleware")

This will enable zeal in any tests that go through your middleware. If you want to enable it in all tests, you need to do a bit more work.

If you use pytest, use a fixture in your conftest.py:

import pytest
from zeal import zeal_context

@pytest.fixture(scope="function", autouse=True)
def use_zeal():
    with zeal_context():
        yield

If you use unittest, add custom test cases and inherit from these rather than directly from Django's test cases:

# In e.g. `myapp/testing/test_cases.py`
from zeal import setup as zeal_setup, teardown as zeal_teardown
import unittest
from django.test import SimpleTestCase, TestCase, TransactionTestCase

class ZealTestMixin(unittest.TestCase):
    def setUp(self, test):
        zeal_setup()
        super().setUp()

    def teardown(self) -> None:
        zeal_teardown()
        return super().teardown(test, err)

class CustomSimpleTestCase(ZealTestMixin, SimpleTestCase):
    pass

class CustomTestCase(ZealTestMixin, TestCase):
    pass

class CustomTransactionTestCase(ZealTestMixin, TransactionTestCase):
    pass

Generic setup

If you also want to detect N+1s in other places not covered here, you can use the setup and teardown functions, or the zeal_context context manager:

from zeal import setup, teardown, zeal_context


def foo():
    setup()
    try:
        # your code goes here
    finally:
        teardown()


@zeal_context()
def bar():
    # your code goes here


def baz():
    with zeal_context():
        # your code goes here

Configuration

By default, any issues detected by zeal will raise a ZealError. If you'd rather log any detected N+1s as warnings, you can set:

ZEAL_RAISE = False

N+1s will be reported when the same query is executed twice. To configure this threshold, set the following in your Django settings.

ZEAL_NPLUSONE_THRESHOLD = 3

To handle false positives, you can temporarily disable zeal in parts of your code using a context manager:

from zeal import zeal_ignore

with zeal_ignore():
    # code in this block will not log/raise zeal errors

If you only want to ignore a specific N+1, you can pass in a list of models/fields to ignore:

with zeal_ignore([{"model": "polls.Question", "field": "options"}]):
    # code in this block will ignore N+1s on Question.options

If you want to listen to N+1 exceptions globally and do something with them, you can listen to the Django signal that zeal emits:

from zeal.signals import nplusone_detected
from django.dispatch import receiver

@receiver(nplusone_detected)
def handle_nplusone(sender, exception):
    # do something

Finally, if you want to ignore N+1 alerts from a specific model/field globally, you can add it to your settings:

ZEAL_ALLOWLIST = [
    {"model": "polls.Question", "field": "options"},

    # you can use fnmatch syntax in the model/field, too
    {"model": "polls.*", "field": "options"},

    # if you don't pass in a field, all N+1s arising from the model will be ignored
    {"model": "polls.Question"},
]

Debugging N+1s

By default, zeal's alerts will tell you the line of your code that executed the same query multiple times. If you'd like to see the full call stack from each time the query was executed, you can set:

ZEAL_SHOW_ALL_CALLERS = True

in your settings. This will give you the full call stack from each time the query was executed.

Comparison to nplusone

zeal borrows heavily from nplusone, but has some differences:

• zeal also detects N+1 caused by using .only() and .defer()
• it lets you configure your own threshold for what constitutes an N+1
• it has slightly more helpful error messages that tell you where the N+1 occurred
• nplusone patches the Django ORM even in production when it's not enabled. zeal does not!
• nplusone appears to be abandoned at this point.
• however, zeal only works with Django, whereas nplusone can also be used with SQLAlchemy.
• zeal does not (yet) detect unused prefetches, but nplusone does.

Contributing

1. First, install uv.
2. Create a virtual env using uv venv and activate it with source .venv/bin/activate.
3. Run make install to install dev dependencies.
4. To run tests, run make test.