fix(squash): add code_field_check() helper for DB-level regex constraint

Adds a code_field_check(field_name, name=...) companion to code_field()
that returns a CheckConstraint using Django's Regex lookup. This enforces
the code regex at the database level (not just via validators), and
Django also runs it as a Python-level validator automatically.

Applied to Collection.collection_code as the first usage.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: kdmccormick

src/openedx_content/applets/collections/models.py

@@ -70,7 +70,7 @@
 from django.db import models
 from django.utils.translation import gettext_lazy as _
 
-from openedx_django_lib.fields import MultiCollationTextField, case_insensitive_char_field, code_field
+from openedx_django_lib.fields import MultiCollationTextField, case_insensitive_char_field, code_field, code_field_check
 from openedx_django_lib.validators import validate_utc_datetime
 
 from ..publishing.models import LearningPackage, PublishableEntity
@@ -179,6 +179,7 @@ class Meta:
                 ],
                 name="oel_coll_uniq_lp_key",
             ),
+            code_field_check("collection_code", name="oel_coll_collection_code_regex"),
         ]
         indexes = [
             models.Index(

src/openedx_content/migrations/0010_collection_oel_coll_collection_code_regex.py

@@ -0,0 +1,20 @@
+# Generated by Django 5.2.13 on 2026-04-16 14:37
+
+import django.db.models.lookups
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('openedx_content', '0009_add_collection_code_regex_validation'),
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+    ]
+
+    operations = [
+        migrations.AddConstraint(
+            model_name='collection',
+            constraint=models.CheckConstraint(condition=django.db.models.lookups.Regex(models.F('collection_code'), '^[a-zA-Z0-9\\-\\_\\.]+\\Z'), name='oel_coll_collection_code_regex', violation_error_message='Enter a valid "code name" consisting of letters, numbers, underscores, hyphens, or periods.'),
+        ),
+    ]

src/openedx_django_lib/fields.py

@@ -18,6 +18,7 @@
 
 from django.core.validators import RegexValidator
 from django.db import models
+from django.db.models.lookups import Regex
 from django.utils.translation import gettext_lazy as _
 
 from .collations import MultiCollationMixin
@@ -121,9 +122,17 @@ def immutable_uuid_field() -> models.UUIDField:
 CODE_REGEX = re.compile(r"^[a-zA-Z0-9\-\_\.]+\Z")
 
 
+_CODE_VIOLATION_MSG = _(
+    'Enter a valid "code name" consisting of letters, numbers, underscores, hyphens, or periods.'
+)
+
+
 def code_field(**kwargs) -> MultiCollationCharField:
     """
     Field to hold a 'code', i.e. a slug-like local identifier.
+
+    Use together with :func:`code_field_check` to enforce the same regex at
+    the database level via a ``CheckConstraint``.
     """
     return case_sensitive_char_field(
         max_length=255,
@@ -132,14 +141,37 @@ def code_field(**kwargs) -> MultiCollationCharField:
             RegexValidator(
                 CODE_REGEX,
                 # Translators: "letters" means latin letters: a-z and A-Z.
-                _('Enter a valid "code name" consisting of letters, numbers, underscores, hyphens, or periods.'),
+                _CODE_VIOLATION_MSG,
                 "invalid",
             ),
         ],
         **kwargs,
     )
 
 
+def code_field_check(field_name: str, *, name: str) -> models.CheckConstraint:
+    """
+    Return a ``CheckConstraint`` that enforces :data:`CODE_REGEX` at the DB level.
+
+    Django validators (used by :func:`code_field`) are not called on ``.save()``
+    or ``.update()``.  Adding this constraint ensures the regex is also enforced
+    by the database itself, and Django will additionally run it as a Python-level
+    validator automatically.
+
+    Usage::
+
+        class Meta:
+            constraints = [
+                code_field_check("my_code_field", name="myapp_mymodel_my_code_field_regex"),
+            ]
+    """
+    return models.CheckConstraint(
+        condition=Regex(models.F(field_name), CODE_REGEX.pattern),
+        name=name,
+        violation_error_message=_CODE_VIOLATION_MSG,
+    )
+
+
 def key_field(**kwargs) -> MultiCollationCharField:
     """
     Externally created Identifier fields.