Fixed #28862 -- Disabled optimization of AlterFooTogether and RemoveField.

AlterFooTogether operations cannot be swapped with RemoveField operations on
the same model as they could be removing the the same field as well.

Since AlterFooTogether operations don't track what their previous value was,
it's impossible to determine whether or not the optimization is safe so the
only way to proceed is to disable the optimization.

Thanks Ramiro Morales for the in-depth analysis of the issue.

Refs #24828

Author: charettes

django/db/migrations/operations/models.py

@@ -449,10 +449,11 @@ def reduce(self, operation, app_label=None):
 
 class FieldRelatedOptionOperation(ModelOptionOperation):
     def reduce(self, operation, app_label=None):
-        if (isinstance(operation, FieldOperation) and
-                self.name_lower == operation.model_name_lower and
-                not self.references_field(operation.model_name, operation.name)):
-            return [operation, self]
+        if isinstance(operation, FieldOperation) and self.name_lower == operation.model_name_lower:
+            if isinstance(operation, RemoveField):
+                return False
+            if not self.references_field(operation.model_name, operation.name):
+                return [operation, self]
         return super().reduce(operation, app_label=app_label)
 
 

tests/migrations/test_autodetector.py

@@ -1529,10 +1529,10 @@ def test_remove_field_and_foo_together(self):
         )
         # Right number/type of migrations?
         self.assertNumberMigrations(changes, "otherapp", 1)
-        self.assertOperationTypes(changes, "otherapp", 0, ["RemoveField", "AlterUniqueTogether", "AlterIndexTogether"])
-        self.assertOperationAttributes(changes, "otherapp", 0, 0, model_name="book", name="newfield")
-        self.assertOperationAttributes(changes, "otherapp", 0, 1, name="book", unique_together={("author", "title")})
-        self.assertOperationAttributes(changes, "otherapp", 0, 2, name="book", index_together={("author", "title")})
+        self.assertOperationTypes(changes, "otherapp", 0, ["AlterUniqueTogether", "AlterIndexTogether", "RemoveField"])
+        self.assertOperationAttributes(changes, "otherapp", 0, 0, name="book", unique_together={("author", "title")})
+        self.assertOperationAttributes(changes, "otherapp", 0, 1, name="book", index_together={("author", "title")})
+        self.assertOperationAttributes(changes, "otherapp", 0, 2, model_name="book", name="newfield")
 
     def test_rename_field_and_foo_together(self):
         """

tests/migrations/test_commands.py

@@ -1335,7 +1335,7 @@ def test_squashmigrations_optimizes(self):
         out = io.StringIO()
         with self.temporary_migration_module(module="migrations.test_migrations"):
             call_command("squashmigrations", "migrations", "0002", interactive=False, verbosity=1, stdout=out)
-        self.assertIn("Optimized from 8 operations to 3 operations.", out.getvalue())
+        self.assertIn("Optimized from 8 operations to 4 operations.", out.getvalue())
 
     def test_ticket_23799_squashmigrations_no_optimize(self):
         """

tests/migrations/test_optimizer.py

@@ -712,7 +712,7 @@ def _test_create_alter_foo_field(self, alter):
             ],
         )
 
-        self.assertOptimizesTo(
+        self.assertDoesNotOptimize(
             [
                 migrations.CreateModel("Foo", [
                     ("a", models.IntegerField()),
@@ -722,13 +722,6 @@ def _test_create_alter_foo_field(self, alter):
                 alter,
                 migrations.RemoveField("Foo", "c"),
             ],
-            [
-                migrations.CreateModel("Foo", [
-                    ("a", models.IntegerField()),
-                    ("b", models.IntegerField()),
-                ]),
-                alter,
-            ],
         )
 
     def test_create_alter_unique_field(self):