charettes / django_queryset_constraint

A library for writing reliable data invariants in Django.

Django Queryset Constraint

This library enables one to write reliable data invariants, by compiling Django Querysets to database insert/update triggers, via. the migration system.

Motivation

Django has a built-in signal system, which emits signals on various events, for instance model creations, updates and deletions. However these signals are not emmited for queryset operations, and as such cannot be used to maintain data invariants.

An attempt to ratify this was made with the Django Queryset Signals library. While this library comes closer to a reliable solution, it does not succeed, as it is stil possible to break the data invariants by accessing the database directly.

Database Constraint Triggers will effectively protect against all scenarios.

Installation

pip install django_queryset_constraint

Usage

• Add the django_queryset_constraint app to INSTALLED_APPS:

# settings.py
INSTALLED_APPS = {
    'django_queryset_constraint',
    ...
}

Note: This should be done before any apps that will be checked

• Add QuerysetConstraint to constraints to the Meta class of the models to checked:

# models.py
from django.db import models
from django.db.models import Count
from django_queryset_constraint import M, QuerysetConstraint


class Topping(models.Model):
    name = models.CharField(max_length=30)

    def __str__(self):
        return self.name

class PizzaTopping(models.Model):
    class Meta:
        unique_together = ("pizza", "topping")
        constraints = [
            # A pizza with more than 5 toppings gets soggy
            QuerysetConstraint(
                name='At most 5 toppings',
                queryset=M().objects.values(
                    'pizza'
                ).annotate(
                    num_toppings=Count('topping')
                ).filter(
                    num_toppings__gt=5
                ),
            ),
            # This constraint should be self-explanatory for civilized people
            QuerysetConstraint(
                name='No pineapple',
                queryset=M().objects.filter(
                    topping__name="Pineapple"
                )
            ),
        ]

    pizza = models.ForeignKey('Pizza', on_delete=models.CASCADE)
    topping = models.ForeignKey(Topping, on_delete=models.CASCADE)

class Pizza(models.Model):
    name = models.CharField(max_length=30)
    toppings = models.ManyToManyField(Topping, through=PizzaTopping)

    def __str__(self):
        return self.name

• Make migrations: python manage.py makemigrations
• Run migrations: python manage.py migrate

Note: Complex triggers introduce performance overhead.

Support Matrix

This app supports the following combinations of Django and Python:

Django	Python
2.2	3.5, 3.6, 3.7