[1.8.x] Fixed #24291 - Fixed migration ModelState generation with unused swappable models

Swapped out models don't have a _default_manager unless they have
explicitly defined managers. ModelState.from_model() now accounts for
this case and uses an empty list for managers if no explicit managers
are defined and a model is swapped out.

Backport of 15dc8d1c9d3697170a2c59ecaa7a2b4ba58f5990 from master

2 files changed, 74 insertions, 19 deletions

diff --git a/django/db/migrations/state.py b/django/db/migrations/state.py
index fdce3c77c3..7c5f9ed776 100644
--- a/django/db/migrations/state.py
+++ b/django/db/migrations/state.py
@@ -416,25 +416,28 @@ class ModelState(object):
             # instance
             managers[mgr.name] = (mgr.creation_counter, instance)
 
-        default_manager_name = model._default_manager.name
-        # Make sure the default manager is always the first
-        if model._default_manager.use_in_migrations:
-            reconstruct_manager(model._default_manager)
+        if hasattr(model, "_default_manager"):
+            default_manager_name = model._default_manager.name
+            # Make sure the default manager is always the first
+            if model._default_manager.use_in_migrations:
+                reconstruct_manager(model._default_manager)
+            else:
+                # Force this manager to be the first and thus default
+                managers[default_manager_name] = (0, models.Manager())
+            # Sort all managers by their creation counter
+            for _, manager, _ in sorted(model._meta.managers):
+                if manager.name == "_base_manager" or not manager.use_in_migrations:
+                    continue
+                reconstruct_manager(manager)
+            # Sort all managers by their creation counter but take only name and
+            # instance for further processing
+            managers = [
+                (name, instance) for name, (cc, instance) in
+                sorted(managers.items(), key=lambda v: v[1])
+            ]
+            if managers == [(default_manager_name, models.Manager())]:
+                managers = []
         else:
-            # Force this manager to be the first and thus default
-            managers[default_manager_name] = (0, models.Manager())
-        # Sort all managers by their creation counter
-        for _, manager, _ in sorted(model._meta.managers):
-            if manager.name == '_base_manager' or not manager.use_in_migrations:
-                continue
-            reconstruct_manager(manager)
-        # Sort all managers by their creation counter but take only name and
-        # instance for further processing
-        managers = [
-            (name, instance) for name, (cc, instance) in
-            sorted(managers.items(), key=lambda v: v[1])
-        ]
-        if managers == [(default_manager_name, models.Manager())]:
             managers = []
 
         # Construct the new ModelState
diff --git a/tests/migrations/test_state.py b/tests/migrations/test_state.py
index 6d1f739da4..aad50df739 100644
--- a/tests/migrations/test_state.py
+++ b/tests/migrations/test_state.py
@@ -4,7 +4,7 @@ from django.db.migrations.operations import DeleteModel, RemoveField
 from django.db.migrations.state import (
     InvalidBasesError, ModelState, ProjectState, get_related_models_recursive,
 )
-from django.test import SimpleTestCase, TestCase
+from django.test import SimpleTestCase, TestCase, override_settings
 
 from .models import (
     FoodManager, FoodQuerySet, ModelWithCustomBase, NoMigrationFoodManager,
@@ -628,6 +628,58 @@ class ModelStateTests(TestCase):
         with self.assertRaisesMessage(InvalidBasesError, "Cannot resolve bases for [<ModelState: 'app.Model'>]"):
             project_state.apps
 
+    @override_settings(TEST_SWAPPABLE_MODEL='migrations.SomeFakeModel')
+    def test_create_swappable(self):
+        """
+        Tests making a ProjectState from an Apps with a swappable model
+        """
+        new_apps = Apps(['migrations'])
+
+        class Author(models.Model):
+            name = models.CharField(max_length=255)
+            bio = models.TextField()
+            age = models.IntegerField(blank=True, null=True)
+
+            class Meta:
+                app_label = 'migrations'
+                apps = new_apps
+                swappable = 'TEST_SWAPPABLE_MODEL'
+
+        author_state = ModelState.from_model(Author)
+        self.assertEqual(author_state.app_label, 'migrations')
+        self.assertEqual(author_state.name, 'Author')
+        self.assertEqual([x for x, y in author_state.fields], ['id', 'name', 'bio', 'age'])
+        self.assertEqual(author_state.fields[1][1].max_length, 255)
+        self.assertEqual(author_state.fields[2][1].null, False)
+        self.assertEqual(author_state.fields[3][1].null, True)
+        self.assertEqual(author_state.options, {'swappable': 'TEST_SWAPPABLE_MODEL'})
+        self.assertEqual(author_state.bases, (models.Model, ))
+        self.assertEqual(author_state.managers, [])
+
+    @override_settings(TEST_SWAPPABLE_MODEL='migrations.SomeFakeModel')
+    def test_custom_manager_swappable(self):
+        """
+        Tests making a ProjectState from unused models with custom managers
+        """
+        new_apps = Apps(['migrations'])
+
+        class Food(models.Model):
+
+            food_mgr = FoodManager('a', 'b')
+            food_qs = FoodQuerySet.as_manager()
+            food_no_mgr = NoMigrationFoodManager('x', 'y')
+
+            class Meta:
+                app_label = "migrations"
+                apps = new_apps
+                swappable = 'TEST_SWAPPABLE_MODEL'
+
+        food_state = ModelState.from_model(Food)
+
+        # The default manager is used in migrations
+        self.assertEqual([name for name, mgr in food_state.managers], ['food_mgr'])
+        self.assertEqual(food_state.managers[0][1].args, ('a', 'b', 1, 2))
+
 
 class RelatedModelsTests(SimpleTestCase):