Fixed #30757 -- Added a system check to ensure max_length fits the longest choice.

django/db/models/fields/__init__.py

@@ -257,6 +257,7 @@ class Field(RegisterLookupMixin):
                 )
             ]
 
+        choice_max_length = 0
         # Expect [group_name, [value, display]]
         for choices_group in self.choices:
             try:
@@ -270,16 +271,32 @@ class Field(RegisterLookupMixin):
                     for value, human_name in group_choices
                 ):
                     break
+                if self.max_length is not None and group_choices:
+                    choice_max_length = max(
+                        choice_max_length,
+                        *(len(value) for value, _ in group_choices if isinstance(value, str)),
+                    )
             except (TypeError, ValueError):
                 # No groups, choices in the form [value, display]
                 value, human_name = group_name, group_choices
                 if not is_value(value) or not is_value(human_name):
                     break
+                if self.max_length is not None and isinstance(value, str):
+                    choice_max_length = max(choice_max_length, len(value))
 
             # Special case: choices=['ab']
             if isinstance(choices_group, str):
                 break
         else:
+            if self.max_length is not None and choice_max_length > self.max_length:
+                return [
+                    checks.Error(
+                        "'max_length' is too small to fit the longest value "
+                        "in 'choices' (%d characters)." % choice_max_length,
+                        obj=self,
+                        id='fields.E009',
+                    ),
+                ]
             return []
 
         return [

docs/ref/checks.txt

@@ -138,6 +138,8 @@ Model fields
 * **fields.E006**: ``db_index`` must be ``None``, ``True`` or ``False``.
 * **fields.E007**: Primary keys must not have ``null=True``.
 * **fields.E008**: All ``validators`` must be callable.
+* **fields.E009**: ``max_length`` is too small to fit the longest value in
+  ``choices`` (``<count>`` characters).
 * **fields.E100**: ``AutoField``\s must set primary_key=True.
 * **fields.E110**: ``BooleanField``\s do not accept null values. *This check
   appeared before support for null values was added in Django 2.1.*

tests/invalid_models_tests/test_ordinary_fields.py

@@ -304,6 +304,32 @@ class CharFieldTests(SimpleTestCase):
 
         self.assertEqual(Model._meta.get_field('field').check(), [])
 
+    def test_choices_in_max_length(self):
+        class Model(models.Model):
+            field = models.CharField(
+                max_length=2, choices=[
+                    ('ABC', 'Value Too Long!'), ('OK', 'Good')
+                ],
+            )
+            group = models.CharField(
+                max_length=2, choices=[
+                    ('Nested', [('OK', 'Good'), ('Longer', 'Longer')]),
+                    ('Grouped', [('Bad', 'Bad')]),
+                ],
+            )
+
+        for name, choice_max_length in (('field', 3), ('group', 6)):
+            with self.subTest(name):
+                field = Model._meta.get_field(name)
+                self.assertEqual(field.check(), [
+                    Error(
+                        "'max_length' is too small to fit the longest value "
+                        "in 'choices' (%d characters)." % choice_max_length,
+                        obj=field,
+                        id='fields.E009',
+                    ),
+                ])
+
     def test_bad_db_index_value(self):
         class Model(models.Model):
             field = models.CharField(max_length=10, db_index='bad')