[1.8.x] Fixed #24895 -- Fixed loading a pair of squashed migrations with a dependency.

Backport of 84522c0d16 from master.

django/db/migrations/loader.py

@@ -222,15 +222,29 @@ class MigrationLoader(object):
                 for child_key in reverse_dependencies.get(replaced, set()):
                     if child_key in migration.replaces:
                         continue
-                    # child_key may appear in a replacement
+                    # List of migrations whose dependency on `replaced` needs
+                    # to be updated to a dependency on `key`.
+                    to_update = []
+                    # Child key may itself be replaced, in which case it might
+                    # not be in `normal` anymore (depending on whether we've
+                    # processed its replacement yet). If it's present, we go
+                    # ahead and update it; it may be deleted later on if it is
+                    # replaced, but there's no harm in updating it regardless.
+                    if child_key in normal:
+                        to_update.append(normal[child_key])
+                    # If the child key is replaced, we update its replacement's
+                    # dependencies too, if necessary. (We don't know if this
+                    # replacement will actually take effect or not, but either
+                    # way it's OK to update the replacing migration).
                     if child_key in reverse_replacements:
-                        for replaced_child_key in reverse_replacements[child_key]:
-                            if replaced in replacing[replaced_child_key].dependencies:
-                                replacing[replaced_child_key].dependencies.remove(replaced)
-                                replacing[replaced_child_key].dependencies.append(key)
-                    else:
-                        normal[child_key].dependencies.remove(replaced)
-                        normal[child_key].dependencies.append(key)
+                        for replaces_child_key in reverse_replacements[child_key]:
+                            if replaced in replacing[replaces_child_key].dependencies:
+                                to_update.append(replacing[replaces_child_key])
+                    # Actually perform the dependency update on all migrations
+                    # that require it.
+                    for migration_needing_update in to_update:
+                        migration_needing_update.dependencies.remove(replaced)
+                        migration_needing_update.dependencies.append(key)
             normal[key] = migration
             # Mark the replacement as applied if all its replaced ones are
             if all(applied_statuses):

docs/releases/1.8.3.txt

@@ -43,3 +43,7 @@ Bugfixes
 
 * Allowed using ``choices`` longer than 1 day with ``DurationField``
   (:ticket:`24897`).
+
+* Fixed a crash when loading squashed migrations from two apps with a
+  dependency between them, where the dependent app's replaced migrations are
+  partially applied (:ticket:`24895`).

tests/migrations/test_loader.py

@@ -259,12 +259,37 @@ class LoaderTests(TestCase):
         loader.build_graph()
 
         plan = set(loader.graph.forwards_plan(('app1', '4_auto')))
-        expected_plan = set([
+        expected_plan = {
             ('app1', '4_auto'),
             ('app1', '2_squashed_3'),
             ('app2', '1_squashed_2'),
             ('app1', '1_auto')
-        ])
+        }
+        self.assertEqual(plan, expected_plan)
+
+    @override_settings(MIGRATION_MODULES={
+        "app1": "migrations.test_migrations_squashed_complex_multi_apps.app1",
+        "app2": "migrations.test_migrations_squashed_complex_multi_apps.app2",
+    })
+    @modify_settings(INSTALLED_APPS={'append': [
+        "migrations.test_migrations_squashed_complex_multi_apps.app1",
+        "migrations.test_migrations_squashed_complex_multi_apps.app2",
+    ]})
+    def test_loading_squashed_complex_multi_apps_partially_applied(self):
+        loader = MigrationLoader(connection)
+        recorder = MigrationRecorder(connection)
+        recorder.record_applied('app1', '1_auto')
+        recorder.record_applied('app1', '2_auto')
+        loader.build_graph()
+
+        plan = set(loader.graph.forwards_plan(('app1', '4_auto')))
+        plan = plan - loader.applied_migrations
+        expected_plan = {
+            ('app1', '4_auto'),
+            ('app1', '3_auto'),
+            ('app2', '1_squashed_2'),
+        }
+
         self.assertEqual(plan, expected_plan)
 
     @override_settings(MIGRATION_MODULES={"migrations": "migrations.test_migrations_squashed_erroneous"})