django-pgconstraints

Declarative PostgreSQL constraint triggers for Django, with foreign-key traversal that the built-in constraints can't express.

UniqueConstraint, CheckConstraint, and GeneratedField in Django 6.0 compile down to plain PostgreSQL constraints — so they can only reference columns on the same table. django-pgconstraints provides three trigger-based classes that accept F("related__field") expressions and compile to PL/pgSQL triggers via django-pgtrigger. Their API mirrors the Django equivalents to keep the mental model familiar.

from django.db.models import F, Q
from django_pgconstraints import CheckConstraintTrigger

class OrderLine(models.Model):
    product = models.ForeignKey(Product, on_delete=models.CASCADE)
    quantity = models.IntegerField()

    class Meta:
        triggers = [
            CheckConstraintTrigger(
                condition=Q(quantity__lte=F("product__stock")),
                name="orderline_qty_lte_stock",
            ),
        ]

What's in the box

• UniqueConstraintTrigger — unique constraint whose fields= accepts FK chains ("book__author"). Also supports Django expressions, partial conditions, nulls_distinct, and deferred timing.
• CheckConstraintTrigger — check constraint whose Q can cross foreign keys (Q(quantity__lte=F("product__stock"))).
• GeneratedFieldTrigger — a computed column whose expression can reference related rows. The package automatically installs reverse triggers so the value stays in sync when that related data changes, and piggybacks RETURNING on save() / bulk_create() so the computed value lands on the Python instance with no extra query.

Each trigger also integrates with Django's Model.full_clean() so ORM-level validation still catches violations before they hit the database, and each accepts violation_error_code / violation_error_message so the errors raised by full_clean() match your application's error-handling conventions.

Install

pip install django-pgconstraints
# or
uv add django-pgconstraints

Add pgtrigger and django_pgconstraints to INSTALLED_APPS:

INSTALLED_APPS = [
    ...
    "pgtrigger",
    "django_pgconstraints",
    ...
]

Triggers go in Meta.triggers (a django-pgtrigger extension), not Meta.constraints. The package ships a system check (pgconstraints.E001) that flags the common mistake.

Requirements

• Python 3.14+
• Django 6.0
• PostgreSQL (any currently supported version)
• django-pgtrigger 4.17+

Documentation

Full documentation at oliverhaas.github.io/django-pgconstraints.

Runnable examples live in examples/:

• examples/simple/ — cross-publisher unique chapter names
• examples/full/ — inventory + purchase orders exercising all three triggers

License

MIT