django-pgtrigger provides primitives for configuring Postgres triggers on Django models.

Triggers can solve a wide variety of database-level problems more elegantly and reliably than in the application-level of Django. Here are some common problems that can be solved with triggers, many of which we later show how to solve in the docs:

1. Protecting updates and deletes or rows or columns (pgtrigger.Protect).
2. Soft deleting models by setting a field to a value on delete (pgtrigger.SoftDelete).
3. Tracking changes to models or columns change, or when specific conditions happen (django-pghistory uses django-pgtrigger to do this).
4. Keeping fields in sync with other fields.
5. Ensuring that engineers use an official interface (e.g. engineers must use User.objects.create_user and not User.objects.create).
6. Only allowing a status field of a model to transition through certain states (pgtrigger.FSM).

Install django-pgtrigger with pip install django-pgtrigger and add pgtrigger to settings.INSTALLED_APPS.

Models are decorated with @pgtrigger.register and supplied with pgtrigger.Trigger objects. If you don't have access to the model definition, you can still call pgtrigger.register programmatically.

Users declare the plpgsql code manually in a pgtrigger.Trigger object or can use the derived triggers in django-pgtrigger that implement common scenarios. For example, pgtrigger.Protect can protect operations on a model, such as deletions:

from django.db import models
import pgtrigger


@pgtrigger.register(pgtrigger.Protect(operation=pgtrigger.Delete))
class CannotBeDeletedModel(models.Model):
    """This model cannot be deleted!"""

django-pgtrigger aims to alleviate the boilerplate of triggers and having to write raw SQL by using common Django idioms. For example, users can use pgtrigger.Q and pgtrigger.F objects to conditionally execute triggers based on the OLD and NEW row being modified. For example, let's only protect deletes against "active" rows of a model:

from django.db import models
import pgtrigger


@pgtrigger.register(
    pgtrigger.Protect(
        operation=pgtrigger.Delete,
        # Protect deletes when the OLD row of the trigger is still active
        condition=pgtrigger.Q(old__is_active=True)
    )
)
class CannotBeDeletedModel(models.Model):
    """Active model object cannot be deleted!"""
    is_active = models.BooleanField(default=True)

Combining pgtrigger.Q, pgtrigger.F, and derived pgtrigger.Trigger objects can solve a wide array of Django problems without ever having to write raw SQL. Users, however, can still customize triggers and write as much raw SQL as needed for their use case.

For a complete run-through of django-pgtrigger and all derived triggers (along with a trigger cookbook!), read the pgtrigger docs. The docs have a full tutorial of how to configure triggers and lots of code examples.

After you have gone through the tutorial in the docs, check out https://wesleykendall.github.io/django-pgtrigger-tutorial/, which is an interactive tutorial written for a Django meetup talk about django-pgtrigger.

View the django-pgtrigger docs here.

Install django-pgtrigger with:

pip3 install django-pgtrigger

After this, add pgtrigger to the INSTALLED_APPS setting of your Django project.

For information on setting up django-pgtrigger for development and contributing changes, view CONTRIBUTING.rst.

• @wesleykendall (Wes Kendall)