CheckConstraintTrigger

A trigger-based check constraint whose Q can reference columns on related models via F("rel__field"). The API mirrors Django's CheckConstraint so the parameters feel familiar.

from django_pgconstraints import CheckConstraintTrigger

Signature

CheckConstraintTrigger(
    *,
    condition: Q,
    violation_error_code: str | None = None,
    violation_error_message: str | None = None,
    name: str,
)

The parameter name matches Django's CheckConstraint: condition=, not check=.

Local conditions

For same-table conditions it works exactly like CheckConstraint:

class Product(models.Model):
    stock = models.IntegerField(default=0)

    class Meta:
        triggers = [
            CheckConstraintTrigger(
                condition=Q(stock__gte=0),
                name="product_stock_non_negative",
            ),
        ]

Foreign-key traversal

Where CheckConstraintTrigger earns its keep — the right-hand side of a lookup can live on a related model:

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",
            ),
        ]

Each __ hop in the F() reference compiles to a nested subquery, so arbitrary-depth chains (F("order__customer__tier")) work the same way.

Multiple checks on the same model

Each check is its own trigger — list as many as you need:

class OrderLine(models.Model):
    class Meta:
        triggers = [
            CheckConstraintTrigger(
                condition=Q(quantity__gt=0),
                name="orderline_qty_positive",
            ),
            CheckConstraintTrigger(
                condition=Q(quantity__lte=F("product__stock")),
                name="orderline_qty_lte_stock",
            ),
        ]

Validation

CheckConstraintTrigger.validate() participates in full_clean():

• For a condition whose F() refs are all same-table, it falls through to Q.check() and raises ValidationError on the instance.
• For conditions that traverse foreign keys, Python-level evaluation is skipped — the trigger is the sole authority and the check happens at save() time.

Both paths honour violation_error_code and violation_error_message. The message supports %(name)s interpolation for the trigger name:

CheckConstraintTrigger(
    condition=Q(quantity__gt=0),
    violation_error_code="invalid_quantity",
    violation_error_message="%(name)s: quantity must be positive.",
    name="orderline_qty_positive",
)

Database errors

Violations raise IntegrityError with PostgreSQL error code 23514 (check_violation), same as a plain CheckConstraint.