[1.7.x] Fixed #24037 -- Prevented data loss possibility when changing Meta.managed.

The migrations autodetector now issues AlterModelOptions operations for
Meta.managed changes instead of DeleteModel + CreateModel.

Thanks iambibhas for the report and Simon and Markus for review.

Backport of 061caa5b386681dc7bdef16918873043224a299c from master

4 files changed, 43 insertions, 17 deletions

diff --git a/django/db/migrations/autodetector.py b/django/db/migrations/autodetector.py
index 99951cdae5..b323182df9 100644
--- a/django/db/migrations/autodetector.py
+++ b/django/db/migrations/autodetector.py
@@ -454,8 +454,9 @@ class MigrationAutodetector(object):
         We also defer any model options that refer to collections of fields
         that might be deferred (e.g. unique_together, index_together).
         """
-        added_models = set(self.new_model_keys) - set(self.old_model_keys)
-        added_unmanaged_models = set(self.new_unmanaged_keys) - set(self.old_unmanaged_keys)
+        old_keys = set(self.old_model_keys).union(self.old_unmanaged_keys)
+        added_models = set(self.new_model_keys) - old_keys
+        added_unmanaged_models = set(self.new_unmanaged_keys) - old_keys
         models = chain(
             sorted(added_models, key=self.swappable_first_key, reverse=True),
             sorted(added_unmanaged_models, key=self.swappable_first_key, reverse=True)
@@ -630,19 +631,14 @@ class MigrationAutodetector(object):
         We also bring forward removal of any model options that refer to
         collections of fields - the inverse of generate_created_models().
         """
-        deleted_models = set(self.old_model_keys) - set(self.new_model_keys)
-        deleted_unmanaged_models = set(self.old_unmanaged_keys) - set(self.new_unmanaged_keys)
+        new_keys = set(self.new_model_keys).union(self.new_unmanaged_keys)
+        deleted_models = set(self.old_model_keys) - new_keys
+        deleted_unmanaged_models = set(self.old_unmanaged_keys) - new_keys
         models = chain(sorted(deleted_models), sorted(deleted_unmanaged_models))
         for app_label, model_name in models:
             model_state = self.from_state.models[app_label, model_name]
             model = self.old_apps.get_model(app_label, model_name)
             if not model._meta.managed:
-                self.add_operation(
-                    app_label,
-                    operations.DeleteModel(
-                        name=model_state.name,
-                    ),
-                )
                 # Skip here, no need to handle fields for unmanaged models
                 continue
 
@@ -952,7 +948,18 @@ class MigrationAutodetector(object):
         makes an operation to represent them in state changes (in case Python
         code in migrations needs them)
         """
-        models_to_check = self.kept_model_keys.union(self.kept_proxy_keys).union(self.kept_unmanaged_keys)
+        models_to_check = self.kept_model_keys.union(
+            self.kept_proxy_keys
+        ).union(
+            self.kept_unmanaged_keys
+        ).union(
+            # unmanaged converted to managed
+            set(self.old_unmanaged_keys).intersection(self.new_model_keys)
+        ).union(
+            # managed converted to unmanaged
+            set(self.old_model_keys).intersection(self.new_unmanaged_keys)
+        )
+
         for app_label, model_name in sorted(models_to_check):
             old_model_name = self.renamed_models.get((app_label, model_name), model_name)
             old_model_state = self.from_state.models[app_label, old_model_name]
diff --git a/django/db/migrations/operations/models.py b/django/db/migrations/operations/models.py
index 75053474a9..bb7f11d90d 100644
--- a/django/db/migrations/operations/models.py
+++ b/django/db/migrations/operations/models.py
@@ -356,6 +356,7 @@ class AlterModelOptions(Operation):
     # Model options we want to compare and preserve in an AlterModelOptions op
     ALTER_OPTION_KEYS = [
         "get_latest_by",
+        "managed",
         "ordering",
         "permissions",
         "default_permissions",
diff --git a/docs/releases/1.7.2.txt b/docs/releases/1.7.2.txt
index 179bd5f7db..309aa736ad 100644
--- a/docs/releases/1.7.2.txt
+++ b/docs/releases/1.7.2.txt
@@ -162,3 +162,8 @@ Bugfixes
 * Added ``datetime.time`` support to migrations questioner (:ticket:`23998`).
 
 * Fixed admindocs crash on apps installed as eggs (:ticket:`23525`).
+
+* Changed migrations autodetector to generate an ``AlterModelOptions`` operation
+  instead of ``DeleteModel`` and ``CreateModel`` operations when changing
+  ``Meta.managed``. This prevents data loss when changing ``managed`` from
+  ``False`` to ``True`` and vice versa (:ticket:`24037`).
diff --git a/tests/migrations/test_autodetector.py b/tests/migrations/test_autodetector.py
index 38771f16ac..8beda8e552 100644
--- a/tests/migrations/test_autodetector.py
+++ b/tests/migrations/test_autodetector.py
@@ -1021,7 +1021,7 @@ class AutodetectorTests(TestCase):
         # The field name the FK on the book model points to
         self.assertEqual(changes['otherapp'][0].operations[0].fields[2][1].rel.field_name, 'pk_field')
 
-    def test_unmanaged(self):
+    def test_unmanaged_create(self):
         """Tests that the autodetector correctly deals with managed models."""
         # First, we test adding an unmanaged model
         before = self.make_project_state([self.author_empty])
@@ -1031,9 +1031,10 @@ class AutodetectorTests(TestCase):
         # Right number/type of migrations?
         self.assertNumberMigrations(changes, 'testapp', 1)
         self.assertOperationTypes(changes, 'testapp', 0, ["CreateModel"])
-        self.assertOperationAttributes(changes, 'testapp', 0, 0, name="AuthorUnmanaged")
-        self.assertEqual(changes['testapp'][0].operations[0].options['managed'], False)
+        self.assertOperationAttributes(changes, 'testapp', 0, 0,
+            name="AuthorUnmanaged", options={"managed": False})
 
+    def test_unmanaged_to_managed(self):
         # Now, we test turning an unmanaged model into a managed model
         before = self.make_project_state([self.author_empty, self.author_unmanaged])
         after = self.make_project_state([self.author_empty, self.author_unmanaged_managed])
@@ -1041,9 +1042,21 @@ class AutodetectorTests(TestCase):
         changes = autodetector._detect_changes()
         # Right number/type of migrations?
         self.assertNumberMigrations(changes, 'testapp', 1)
-        self.assertOperationTypes(changes, 'testapp', 0, ["DeleteModel", "CreateModel"])
-        self.assertOperationAttributes(changes, 'testapp', 0, 0, name="AuthorUnmanaged")
-        self.assertOperationAttributes(changes, 'testapp', 0, 1, name="AuthorUnmanaged")
+        self.assertOperationTypes(changes, 'testapp', 0, ["AlterModelOptions"])
+        self.assertOperationAttributes(changes, 'testapp', 0, 0,
+            name="authorunmanaged", options={})
+
+    def test_managed_to_unmanaged(self):
+        # Now, we turn managed to unmanaged.
+        before = self.make_project_state([self.author_empty, self.author_unmanaged_managed])
+        after = self.make_project_state([self.author_empty, self.author_unmanaged])
+        autodetector = MigrationAutodetector(before, after)
+        changes = autodetector._detect_changes()
+        # Right number/type of migrations?
+        self.assertNumberMigrations(changes, 'testapp', 1)
+        self.assertOperationTypes(changes, "testapp", 0, ["AlterModelOptions"])
+        self.assertOperationAttributes(changes, "testapp", 0, 0,
+            name="authorunmanaged", options={"managed": False})
 
     def test_unmanaged_custom_pk(self):
         """