Added inline inline_comment_sql() and modified _column_sql().

Per comment django#14463 (comment).

django/db/backends/base/schema.py

@@ -280,7 +280,7 @@ def table_sql(self, model):
         if (
             model._meta.db_table_comment
             and self.connection.features.supports_comments_inline
-            and (comment_sql := self._comment_sql(model._meta.db_table_comment))
+            and (comment_sql := self.inline_comment_sql(model._meta.db_table_comment))
         ):
             sql += comment_sql
 
@@ -444,15 +444,6 @@ def quote_value(self, value):
         """
         raise NotImplementedError()
 
-    def quote_comment_value(self, value):
-        """
-        Each database handles '', None differently.
-        Postgres: '', None -> NULL
-        Mysql: '', None -> ''
-        Oracle: '', None -> ''
-        """
-        return self.quote_value(value or "")
-
     # Actions
 
     def create_model(self, model):
@@ -475,7 +466,7 @@ def create_model(self, model):
                 self.sql_alter_table_comment
                 % {
                     "table": self.quote_name(model._meta.db_table),
-                    "comment": self.quote_comment_value(model._meta.db_table_comment),
+                    "comment": self._comment_sql(model._meta.db_table_comment),
                 },
                 [],
             )
@@ -492,7 +483,7 @@ def create_model(self, model):
                     % {
                         "table": self.quote_name(model._meta.db_table),
                         "column": self.quote_name(field.column),
-                        "comment": self.quote_comment_value(field.db_comment),
+                        "comment": self._comment_sql(field.db_comment),
                     },
                     [],
                 )
@@ -673,7 +664,7 @@ def alter_db_table_comment(self, model, old_db_table_comment, new_db_table_comme
             self.sql_alter_table_comment
             % {
                 "table": self.quote_name(model._meta.db_table),
-                "comment": self.quote_comment_value(new_db_table_comment),
+                "comment": self._comment_sql(new_db_table_comment),
             }
         )
 
@@ -767,7 +758,7 @@ def add_field(self, model, field):
                 % {
                     "table": self.quote_name(model._meta.db_table),
                     "column": self.quote_name(field.column),
-                    "comment": self.quote_comment_value(field.db_comment),
+                    "comment": self._comment_sql(field.db_comment),
                 },
                 [],
             )
@@ -1250,9 +1241,8 @@ def _alter_column_null_sql(self, model, old_field, new_field):
                 if new_field.null
                 else self.sql_alter_column_not_null
             )
-            if (
-                self.connection.features.supports_comments_inline
-                and (comment_sql := self._comment_sql(new_field.db_comment))
+            if self.connection.features.supports_comments_inline and (
+                comment_sql := self.inline_comment_sql(new_field.db_comment)
             ):
                 sql += comment_sql
             return (
@@ -1340,14 +1330,21 @@ def _alter_column_comment_sql(self, model, new_field, new_type, new_db_comment):
                 % {
                     "table": self.quote_name(model._meta.db_table),
                     "column": self.quote_name(new_field.column),
-                    "comment": self.quote_comment_value(new_db_comment),
+                    "comment": self._comment_sql(new_db_comment),
                 },
                 [],
             )
         ]
 
     def _comment_sql(self, comment):
-        return ""
+        return self.quote_value(comment or "")
+
+    def inline_comment_sql(self, value):
+        """Only used for backends which support inline comments."""
+        raise NotImplementedError(
+            "Subclasses of BaseDatabaseSchemaEditor for backends which support inline "
+            "comments must provide a inline_comment_sql() method."
+        )
 
     def _alter_many_to_many(self, model, old_field, new_field, strict):
         """Alter M2Ms to repoint their to= endpoints."""

django/db/backends/mysql/schema.py

@@ -35,7 +35,7 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
 
     sql_alter_table_comment = "ALTER TABLE %(table)s COMMENT = %(comment)s"
     sql_alter_column_comment = (
-        "ALTER TABLE %(table)s MODIFY %(column)s %(type)s COMMENT %(comment)s"
+        "ALTER TABLE %(table)s MODIFY %(column)s %(type)s%(comment)s"
     )
 
     @property
@@ -60,9 +60,8 @@ def sql_rename_column(self):
 
     def column_sql(self, model, field, include_default=False):
         sql, params = super().column_sql(model, field, include_default)
-
         if field.db_comment and not field.many_to_many:
-            sql += self._comment_sql(field.db_comment)
+            sql += self.inline_comment_sql(field.db_comment)
         return sql, params
 
     def quote_value(self, value):
@@ -239,7 +238,7 @@ def _alter_column_type_sql(
             "column": self.quote_name(new_field.column),
             "type": new_type,
         }
-        sql += self._comment_sql(new_field.db_comment)
+        sql += self.inline_comment_sql(new_field.db_comment)
         return (sql, []), []
 
     def _rename_field_sql(self, table, old_field, new_field, new_type):
@@ -254,11 +253,12 @@ def _alter_column_comment_sql(self, model, new_field, new_type, new_db_comment):
                     "table": self.quote_name(model._meta.db_table),
                     "column": self.quote_name(new_field.column),
                     "type": self._set_field_new_type_null_status(new_field, new_type),
-                    "comment": self.quote_comment_value(new_db_comment),
+                    "comment": self.inline_comment_sql(new_db_comment),
                 },
                 [],
             )
         ]
 
-    def _comment_sql(self, comment):
-        return " COMMENT " + self.quote_comment_value(comment)
+    def inline_comment_sql(self, comment):
+        comment = self._comment_sql(comment)
+        return f" COMMENT {comment}"

tests/backends/base/test_schema.py

@@ -1,4 +1,4 @@
-from django.db import models
+from django.db import connection, models
 from django.db.backends.base.schema import BaseDatabaseSchemaEditor
 from django.test import SimpleTestCase
 
@@ -17,3 +17,12 @@ def _get_default(self):
 
         field = MyCharField(max_length=1, default=MyStr)
         self.assertEqual(BaseDatabaseSchemaEditor._effective_default(field), MyStr)
+
+    def test_inline_comment_sql(self):
+        schema_editor = BaseDatabaseSchemaEditor(connection=connection)
+        msg = (
+            "Subclasses of BaseDatabaseSchemaEditor for backends which support inline "
+            "comments must provide a inline_comment_sql() method."
+        )
+        with self.assertRaisesMessage(NotImplementedError, msg):
+            schema_editor.inline_comment_sql(None)