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