[pull] main from django:main

AUTHORS

@@ -1136,6 +1136,7 @@ answer newbie questions, and generally made Django that much better:
     Yann Fouillat <gagaro42@gmail.com>
     Yann Malet
     Yash Jhunjhunwala
+    Yassin Bahri <https://github.com/yassinbahri>
     Yasushi Masuda <whosaysni@gmail.com>
     ye7cakf02@sneakemail.com
     ymasuda@ethercube.com

django/db/backends/base/features.py

@@ -377,6 +377,8 @@ class BaseDatabaseFeatures:
     supports_comments = False
     # Does the backend support column comments in ADD COLUMN statements?
     supports_comments_inline = False
+    # Does the backend support changing column comments without altering type?
+    supports_independent_comment_alteration = False
 
     # Does the backend support stored generated columns?
     supports_stored_generated_columns = False

django/db/backends/base/schema.py

@@ -1084,6 +1084,9 @@ def _alter_field(
             and (not new_field.db_index or new_field.unique)
         ):
             # Find the index for this field
+            meta_constraint_names = {
+                constraint.name for constraint in model._meta.constraints
+            }
             meta_index_names = {index.name for index in model._meta.indexes}
             # Retrieve only BTREE indexes since this is what's created with
             # db_index=True.
@@ -1092,7 +1095,7 @@ def _alter_field(
                 [old_field.column],
                 index=True,
                 type_=Index.suffix,
-                exclude=meta_index_names,
+                exclude=meta_constraint_names | meta_index_names,
             )
             for index_name in index_names:
                 # The only way to check if an index was created with
@@ -1151,13 +1154,23 @@ def _alter_field(
             or (
                 self.connection.features.supports_comments
                 and old_field.db_comment != new_field.db_comment
+                and not self.connection.features.supports_independent_comment_alteration
             )
         ):
             fragment, other_actions = self._alter_column_type_sql(
                 model, old_field, new_field, new_type, old_collation, new_collation
             )
             actions.append(fragment)
             post_actions.extend(other_actions)
+        elif (
+            self.connection.features.supports_comments
+            and old_field.db_comment != new_field.db_comment
+            and self.connection.features.supports_independent_comment_alteration
+        ):
+            sql, params = self._alter_column_comment_sql(
+                model, new_field, new_type, new_field.db_comment
+            )
+            post_actions.append((sql, params))
 
         if new_field.has_db_default():
             if (

django/db/backends/mysql/schema.py

@@ -226,7 +226,7 @@ def _rename_field_sql(self, table, old_field, new_field, new_type):
         return super()._rename_field_sql(table, old_field, new_field, new_type)
 
     def _alter_column_comment_sql(self, model, new_field, new_type, new_db_comment):
-        # Comment is alter when altering the column type.
+        # Comment is altered when altering the column type.
         return "", []
 
     def _comment_sql(self, comment):

django/db/backends/oracle/features.py

@@ -31,6 +31,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     supports_deferrable_unique_constraints = True
     truncates_names = True
     supports_comments = True
+    supports_independent_comment_alteration = True
     supports_tablespaces = True
     supports_sequence_reset = False
     can_introspect_materialized_views = True

django/db/backends/postgresql/features.py

@@ -25,6 +25,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     has_select_for_no_key_update = True
     can_release_savepoints = True
     supports_comments = True
+    supports_independent_comment_alteration = True
     supports_tablespaces = True
     supports_transactions = True
     can_introspect_materialized_views = True

tests/schema/tests.py

@@ -1111,6 +1111,41 @@ class Meta:
             "generated", self.get_indexes(GeneratedFieldIndexedModel._meta.db_table)
         )
 
+    @isolate_apps("schema")
+    @skipUnlessDBFeature(
+        "supports_stored_generated_columns",
+        "supports_independent_comment_alteration",
+    )
+    def test_alter_generated_field_base_field_comment(self):
+        class GenFieldModelComment(Model):
+            name = CharField(max_length=100)
+            name_lower = GeneratedField(
+                expression=Lower("name"), db_persist=True, output_field=CharField()
+            )
+
+            class Meta:
+                app_label = "schema"
+
+        with connection.schema_editor() as editor:
+            editor.create_model(GenFieldModelComment)
+
+        old_field = GenFieldModelComment._meta.get_field("name")
+        new_field = CharField(max_length=100, db_comment="Super useful comment")
+        new_field.set_attributes_from_name("name")
+        new_field.model = GenFieldModelComment
+        with (
+            connection.schema_editor() as editor,
+            CaptureQueriesContext(connection) as ctx,
+        ):
+            editor.alter_field(GenFieldModelComment, old_field, new_field, strict=True)
+
+        self.assertEqual(len(ctx), 1)
+        self.assertIn("COMMENT ON COLUMN", ctx.captured_queries[0]["sql"])
+        self.assertEqual(
+            self.get_column_comment(GenFieldModelComment._meta.db_table, "name"),
+            "Super useful comment",
+        )
+
     @isolate_apps("schema")
     def test_add_auto_field(self):
         class AddAutoFieldModel(Model):
@@ -4113,6 +4148,42 @@ def test_remove_db_index_doesnt_remove_custom_indexes(self):
         finally:
             AuthorWithIndexedName._meta.indexes = []
 
+    @skipUnlessDBFeature("allows_multiple_constraints_on_same_fields")
+    def test_remove_db_index_doesnt_remove_unique_constraints(self):
+        with connection.schema_editor() as editor:
+            editor.create_model(AuthorCharFieldWithIndex)
+        constraint = UniqueConstraint(
+            fields=["char_field"], name="author_char_field_uniq"
+        )
+        try:
+            AuthorCharFieldWithIndex._meta.constraints = [constraint]
+            with connection.schema_editor() as editor:
+                editor.add_constraint(AuthorCharFieldWithIndex, constraint)
+                db_index_name = editor._create_index_name(
+                    table_name=AuthorCharFieldWithIndex._meta.db_table,
+                    column_names=("char_field",),
+                )
+            old_constraints = self.get_constraints(
+                AuthorCharFieldWithIndex._meta.db_table
+            )
+            self.assertIn(constraint.name, old_constraints)
+            self.assertIn(db_index_name, old_constraints)
+
+            old_field = AuthorCharFieldWithIndex._meta.get_field("char_field")
+            new_field = CharField(max_length=31)
+            new_field.set_attributes_from_name("char_field")
+            with connection.schema_editor() as editor:
+                editor.alter_field(
+                    AuthorCharFieldWithIndex, old_field, new_field, strict=True
+                )
+            new_constraints = self.get_constraints(
+                AuthorCharFieldWithIndex._meta.db_table
+            )
+            self.assertNotIn(db_index_name, new_constraints)
+            self.assertIn(constraint.name, new_constraints)
+        finally:
+            AuthorCharFieldWithIndex._meta.constraints = []
+
     def test_order_index(self):
         """
         Indexes defined with ordering (ASC/DESC) defined on column
@@ -5103,12 +5174,19 @@ def test_add_db_comment_and_default_charfield(self):
     def test_alter_db_comment(self):
         with connection.schema_editor() as editor:
             editor.create_model(Author)
-        # Add comment.
         old_field = Author._meta.get_field("name")
         new_field = CharField(max_length=255, db_comment="Custom comment")
         new_field.set_attributes_from_name("name")
-        with connection.schema_editor() as editor:
+        with (
+            connection.schema_editor() as editor,
+            CaptureQueriesContext(connection) as ctx,
+        ):
             editor.alter_field(Author, old_field, new_field, strict=True)
+        self.assertEqual(len(ctx), 1)
+        if connection.features.supports_independent_comment_alteration:
+            self.assertIn("COMMENT ON COLUMN", ctx.captured_queries[0]["sql"])
+        else:
+            self.assertIn("ALTER TABLE", ctx.captured_queries[0]["sql"])
         self.assertEqual(
             self.get_column_comment(Author._meta.db_table, "name"),
             "Custom comment",