Add system check for null kwarg in GeneratedField definition

[Dai-Tado]

Summary
GeneratedField ignores null=False on MySQL - columns are always nullable

Description
When creating a GeneratedField with null=False on MySQL, the database column is always created as nullable, ignoring the null=False parameter.
Steps to Reproduce

pythonfrom django.db import models

class MyModel(models.Model):
    area = models.GeneratedField(
        .....
        null=False,  # ← Ignored
    )

• The migration file removes null=False from GeneratedField definition
• Even if manually added to migration file, Django doesn't apply NOT NULL constraint to database

[Dai-Tado]

If this is the expected behavior, would it be possible to include this in the official documentation?

[Jacob Walls]

Good idea, I would be happy to accept something similar to what we have for ManyToManyField, e.g.:

django/db/models/fields/generated.py

@@ -30 +30 @@
         self.expression = expression
         self.output_field = output_field
         self.db_persist = db_persist
+        self.has_null = "null" in kwargs
         super().__init__(**kwargs)
 
     @cached_property
@@ -82 +83 @@
             *super().check(**kwargs),
             *self._check_supported(databases),
             *self._check_persistence(databases),
+            *self._check_ignored_options(databases),
         ]
         output_field_clone = self.output_field.clone()
         output_field_clone.model = self.model
@@ -188 +190 @@
                 )
         return errors
 
+    def _check_ignored_options(self, **kwargs):
+        warnings = []
+
+        if self.has_null_arg:
+            warnings.append(
+                checks.Warning(
+                    "null has no effect on GeneratedField.",
+                    obj=self,
+                    id="fields.WXXX",
+                )
+            )
+
+        return warnings
+
     def deconstruct(self):
         name, path, args, kwargs = super().deconstruct()
         del kwargs["blank"]

---

Would you like to submit a PR and add a test? You should be able to riff on the docs & tests for ManyToMany from 011abb7d96c75f6154c15a8a0f997d8c27698679 (#19671).

[Nilesh Pahari]

PR: ​https://github.com/django/django/pull/20421

[Dai-Tado]

Thank you!!

[Jacob Walls <jacobtylerwalls@…>]

In 6025eab:

Fixed #36806 -- Added system check for null kwarg in GeneratedField.

The null argument has no effect on GeneratedField since the nullability
of the column depends on the database and expression used.

[Jacob Walls <jacobtylerwalls@…>]

In 968f3f96:

Refs #36806 -- Removed unnecessary null=True from GeneratedField in test models.

Test coverage is preserved because the tests only verify that generated
fields can produce NULL values when the source field is nullable.

This addresses system check warnings in the test suite after
6025eab3c509b4de922117e16866bbfe0ee99aa6.