Fixed #24900 -- Allowed migrating backward to squashed migrations.

django/db/migrations/executor.py

@@ -40,6 +40,15 @@ class MigrationExecutor:
             # If the migration is already applied, do backwards mode,
             # otherwise do forwards mode.
             elif target in applied:
+                # If the target is missing, it's likely a replaced migration.
+                # Reload the graph without replacements.
+                if (
+                    self.loader.replace_migrations and
+                    target not in self.loader.graph.node_map
+                ):
+                    self.loader.replace_migrations = False
+                    self.loader.build_graph()
+                    return self.migration_plan(targets, clean_start=clean_start)
                 # Don't migrate backwards all the way to the target node (that
                 # may roll back dependencies in other apps that don't need to
                 # be rolled back); instead roll back through target's immediate

tests/migrations/test_commands.py

@@ -980,6 +980,19 @@ class MigrateTests(MigrationTestBase):
                 # Unmigrate everything.
                 call_command('migrate', 'migrations', 'zero', verbosity=0)
 
+    @override_settings(MIGRATION_MODULES={'migrations': 'migrations.test_migrations_squashed'})
+    def test_migrate_backward_to_squashed_migration(self):
+        try:
+            call_command('migrate', 'migrations', '0001_squashed_0002', verbosity=0)
+            self.assertTableExists('migrations_author')
+            self.assertTableExists('migrations_book')
+            call_command('migrate', 'migrations', '0001_initial', verbosity=0)
+            self.assertTableExists('migrations_author')
+            self.assertTableNotExists('migrations_book')
+        finally:
+            # Unmigrate everything.
+            call_command('migrate', 'migrations', 'zero', verbosity=0)
+
     @override_settings(MIGRATION_MODULES={'migrations': 'migrations.test_migrations'})
     def test_migrate_inconsistent_history(self):
         """

tests/migrations/test_executor.py

@@ -104,6 +104,29 @@ class ExecutorTests(MigrationTestBase):
         self.assertTableNotExists("migrations_author")
         self.assertTableNotExists("migrations_book")
 
+    @override_settings(
+        MIGRATION_MODULES={'migrations': 'migrations.test_migrations_squashed'},
+    )
+    def test_migrate_backward_to_squashed_migration(self):
+        executor = MigrationExecutor(connection)
+        try:
+            self.assertTableNotExists('migrations_author')
+            self.assertTableNotExists('migrations_book')
+            executor.migrate([('migrations', '0001_squashed_0002')])
+            self.assertTableExists('migrations_author')
+            self.assertTableExists('migrations_book')
+            executor.loader.build_graph()
+            # Migrate backward to a squashed migration.
+            executor.migrate([('migrations', '0001_initial')])
+            self.assertTableExists('migrations_author')
+            self.assertTableNotExists('migrations_book')
+        finally:
+            # Unmigrate everything.
+            executor = MigrationExecutor(connection)
+            executor.migrate([('migrations', None)])
+            self.assertTableNotExists('migrations_author')
+            self.assertTableNotExists('migrations_book')
+
     @override_settings(MIGRATION_MODULES={"migrations": "migrations.test_migrations_non_atomic"})
     def test_non_atomic_migration(self):
         """
@@ -733,6 +756,7 @@ class FakeLoader:
     def __init__(self, graph, applied):
         self.graph = graph
         self.applied_migrations = applied
+        self.replace_migrations = True
 
 
 class FakeMigration: