GeneratedField with db_comment causes syntax error

[Jason Christa]

Adding a new GeneratedField with the db_comment argument cause invalid SQL to be generated for MySQL (maybe others but that is all I tested). Altering a GeneratedField with a db_comment seems to work fine.

[Simon Charette]

Reproduced with the following test

tests/schema/tests.py

@@ -4861 +4861 @@
             comment,
         )
 
+    @skipUnlessDBFeature("supports_comments")
+    def test_add_db_comment_generated_field(self):
+        comment = "Custom comment"
+        field = GeneratedField(
+            expression=Value(1),
+            db_persist=True,
+            output_field=IntegerField(),
+            db_comment=comment,
+        )
+        field.set_attributes_from_name("volume")
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+            editor.add_field(Author, field)
+        self.assertEqual(
+            self.get_column_comment(Author._meta.db_table, "volume"),
+            comment,
+        )
+
     @skipUnlessDBFeature("supports_comments")
     def test_add_db_comment_and_default_charfield(self):
         comment = "Custom comment with default"

It appears that the problem is due to the order the the COMMENT and GENERATED clause in the generated SQL.

MySQL expects COMMENT to come after GENERATED while we do the opposite

ALTER TABLE `schema_author` ADD COLUMN `volume` integer COMMENT 'Custom comment' GENERATED ALWAYS AS (1) STORED

Crashes while

ALTER TABLE `schema_author` ADD COLUMN `volume` integer GENERATED ALWAYS AS (1) STORED COMMENT 'Custom comment' 

passes.

[Simon Charette]

Jason, If you're interested in submitting a patch it seems that the solution lies in changing the order of the generated SQL in _iter_column_sql.