[1.8.x] Refs #24628 -- Added a second test and a docstring comment to avoid regression.

Backport of 5c085ea7b3 from master.

django/db/migrations/executor.py

@@ -180,6 +180,14 @@ class MigrationExecutor(object):
     def check_replacements(self):
         """
         Mark replacement migrations applied if their replaced set all are.
+
+        We do this unconditionally on every migrate, rather than just when
+        migrations are applied or unapplied, so as to correctly handle the case
+        when a new squash migration is pushed to a deployment that already had
+        all its replaced migrations applied. In this case no new migration will
+        be applied, but we still want to correctly maintain the applied state
+        of the squash migration.
+
         """
         applied = self.recorder.applied_migrations()
         for key, migration in self.loader.replacements.items():

tests/migrations/test_executor.py

@@ -437,6 +437,30 @@ class ExecutorTests(MigrationTestBase):
             recorder.applied_migrations(),
         )
 
+    @override_settings(MIGRATION_MODULES={"migrations": "migrations.test_migrations_squashed"})
+    def test_migrate_marks_replacement_applied_even_if_it_did_nothing(self):
+        """
+        A new squash migration will be marked as applied even if all its
+        replaced migrations were previously already applied.
+
+        Ticket #24628.
+
+        """
+        recorder = MigrationRecorder(connection)
+        # Record all replaced migrations as applied
+        recorder.record_applied("migrations", "0001_initial")
+        recorder.record_applied("migrations", "0002_second")
+        executor = MigrationExecutor(connection)
+        executor.migrate([("migrations", "0001_squashed_0002")])
+
+        # Because 0001 and 0002 are both applied, even though this migrate run
+        # didn't apply anything new, their squashed replacement should be
+        # marked as applied.
+        self.assertIn(
+            ("migrations", "0001_squashed_0002"),
+            recorder.applied_migrations(),
+        )
+
 
 class FakeLoader(object):
     def __init__(self, graph, applied):