CheckConstraint with __regex lookup + choices on the same field crashes full_clean with AssertionError

[davegaeddert]

Summary

When a model field has both choices=... and a CheckConstraint whose check uses __regex on that field, calling full_clean() (or save(), which calls it) on an invalid value raises AssertionError: Field lookups require a model from inside Q.check(), instead of a ValidationError.

The constraint enforces correctly at the DB level — only the client-side validation path crashes. Removing choices from the field makes the regex constraint validate cleanly with a proper ValidationError.

Versions

• plain 0.139.0
• plain-postgres 0.101.0

Reproducer

# app/example/models.py
from plain import postgres
from plain.postgres import types

ROLE_CHOICES = [("admin", "Admin"), ("member", "Member")]

@postgres.register_model
class Thing(postgres.Model):
    role: str = types.TextField(max_length=20, choices=ROLE_CHOICES, default="member")

    model_options = postgres.Options(
        constraints=[
            postgres.CheckConstraint(
                check=postgres.Q(role__regex=r"^(admin|member)$"),
                name="thing_role_valid",
            ),
        ],
    )

# app/example/probe.py
from app.example.models import Thing
t = Thing(role="bogus")
t.full_clean()

$ uv run plain run app/example/probe.py
Traceback (most recent call last):
  ...
  File ".../plain/postgres/base.py", line 819, in validate_constraints
    constraint.validate(model_class, self, exclude=exclude)
  File ".../plain/postgres/constraints.py", line 112, in validate
    if not Q(self.check).check(against):
  File ".../plain/postgres/query_utils.py", line 166, in check
    query.add_q(Q(Coalesce(self, True, output_field=BooleanField())))
  ...
  File ".../plain/postgres/sql/query.py", line 1224, in solve_lookup_type
    assert self.model is not None, "Field lookups require a model"
AssertionError: Field lookups require a model

constraints.py:107-115 catches FieldError but not AssertionError, so the crash propagates.

Expected

A ValidationError should be collected (either from the choices validator, the constraint's violation_error, or both).

Without choices, it works

Same model with choices=ROLE_CHOICES removed:

{'__all__': [ValidationError(['Constraint "thing_role_valid" is violated.'])]}

So the bug is specifically the interaction between choices on the field and a CheckConstraint that does a __regex lookup against that field during Q.check() validation.

Workaround

Drop choices= from the field (rely on the DB CheckConstraint as the source of truth) — or drop the CheckConstraint and rely on choices for Python-side enforcement only. Can't currently have both.

Probable fix area

plain/postgres/query_utils.py — Q.check() builds a Query(None) and resolves the constraint expression as if it were a field lookup. With choices present, something in the resolution path tries to look up the field via the model, hits Query.solve_lookup_type() (plain/postgres/sql/query.py:1224), and asserts. Either:

• Don't recurse into a model-bound lookup when the value is already an annotation in the in-memory Query, or
• Catch AssertionError (and probably broader) in CheckConstraint.validate() and surface a ValidationError, or
• Ensure choice fields don't add a model-bound resolution layer when invoked from Q.check().

[davegaeddert]

Fixed in d13f47d. CheckConstraint.validate() now exits early when any field referenced by self.check isn't in the value map — when choices validation fails first and excludes the field, the constraint check no longer tries to resolve a missing annotation and crash.