Fixed #23474 -- Prevented migrating backwards from unapplying the wrong migrations.

tests/migrations/test_executor.py

@@ -231,3 +231,41 @@ class ExecutorTests(MigrationTestBase):
         executor.migrate([("migrations", None)])
         self.assertTableNotExists("migrations_author")
         self.assertTableNotExists("migrations_tribble")
+
+    @override_settings(
+        MIGRATION_MODULES={
+          "migrations": "migrations.test_migrations_backwards_deps_1",
+          "migrations2": "migrations2.test_migrations_backwards_deps_2",
+        },
+    )
+    def test_backwards_deps(self):
+        """
+        #23474 - Migrating backwards shouldn't cause the wrong migrations to be
+        unapplied.
+
+        Migration dependencies (x -> y === y depends on x):
+        m.0001 -+-> m.0002
+                +-> m2.0001
+
+        1) Migrate m2 to 0001, causing { m.0001, m2.0002 } to be applied.
+        2) Migrate m to 0001. m.0001 has already been applied, so this should
+           be a noop.
+        """
+        executor = MigrationExecutor(connection)
+        executor.migrate([("migrations2", "0001_initial")])
+        try:
+            self.assertTableExists("migrations2_example")
+            # Rebuild the graph to reflect the new DB state
+            executor.loader.build_graph()
+            self.assertEqual(
+                executor.migration_plan([("migrations", "0001_initial")]),
+                [],
+            )
+            executor.migrate([("migrations", "0001_initial")])
+            self.assertTableExists("migrations2_example")
+        finally:
+            # And migrate back to clean up the database
+            executor.loader.build_graph()
+            executor.migrate([("migrations", None)])
+            self.assertTableNotExists("migrations_author")
+            self.assertTableNotExists("migrations_tribble")