Fixed #23101: Prefer doing deletes before creates in autodetector.

Makes declined or missed renames still work (but drop data).

django/db/migrations/autodetector.py

@@ -160,15 +160,16 @@ class MigrationAutodetector(object):
                     self.through_users[through_key] = (app_label, old_model_name, field_name)
 
         # Generate non-rename model operations
-        self.generate_created_models()
         self.generate_deleted_models()
-        self.generate_created_proxies()
+        self.generate_created_models()
         self.generate_deleted_proxies()
+        self.generate_created_proxies()
         self.generate_altered_options()
 
         # Generate field operations
-        self.generate_added_fields()
+        self.generate_renamed_fields()
         self.generate_removed_fields()
+        self.generate_added_fields()
         self.generate_altered_fields()
         self.generate_altered_unique_together()
         self.generate_altered_index_together()
@@ -682,17 +683,17 @@ class MigrationAutodetector(object):
                 ),
             )
 
-    def generate_added_fields(self):
-        # New fields
+    def generate_renamed_fields(self):
+        """
+        Works out renamed fields
+        """
         self.renamed_fields = {}
         for app_label, model_name, field_name in sorted(self.new_field_keys - self.old_field_keys):
             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]
-            new_model_state = self.to_state.models[app_label, model_name]
             field = self.new_apps.get_model(app_label, model_name)._meta.get_field_by_name(field_name)[0]
             # Scan to see if this is actually a rename!
             field_dec = self.deep_deconstruct(field)
-            found_rename = False
             for rem_app_label, rem_model_name, rem_field_name in sorted(self.old_field_keys - self.new_field_keys):
                 if rem_app_label == app_label and rem_model_name == model_name:
                     old_field_dec = self.deep_deconstruct(old_model_state.get_field_by_name(rem_field_name))
@@ -713,10 +714,15 @@ class MigrationAutodetector(object):
                             self.old_field_keys.remove((rem_app_label, rem_model_name, rem_field_name))
                             self.old_field_keys.add((app_label, model_name, field_name))
                             self.renamed_fields[app_label, model_name, field_name] = rem_field_name
-                            found_rename = True
                             break
-            if found_rename:
-                continue
+
+
+    def generate_added_fields(self):
+        """
+        Fields that have been added
+        """
+        for app_label, model_name, field_name in sorted(self.new_field_keys - self.old_field_keys):
+            field = self.new_apps.get_model(app_label, model_name)._meta.get_field_by_name(field_name)[0]
             # Fields that are foreignkeys/m2ms depend on stuff
             dependencies = []
             if field.rel and field.rel.to:

tests/migrations/test_autodetector.py

@@ -278,16 +278,10 @@ class AutodetectorTests(TestCase):
         after = self.make_project_state([self.author_name_renamed])
         autodetector = MigrationAutodetector(before, after, MigrationQuestioner({"ask_rename": True}))
         changes = autodetector._detect_changes()
-        # Right number of migrations?
-        self.assertEqual(len(changes['testapp']), 1)
-        # Right number of actions?
-        migration = changes['testapp'][0]
-        self.assertEqual(len(migration.operations), 1)
-        # Right action?
-        action = migration.operations[0]
-        self.assertEqual(action.__class__.__name__, "RenameField")
-        self.assertEqual(action.old_name, "name")
-        self.assertEqual(action.new_name, "names")
+        # Check
+        self.assertNumberMigrations(changes, 'testapp', 1)
+        self.assertOperationTypes(changes, 'testapp', 0, ["RenameField"])
+        self.assertOperationAttributes(changes, 'testapp', 0, 0, old_name="name", new_name="names")
 
     def test_rename_model(self):
         "Tests autodetection of renamed models"
@@ -731,21 +725,12 @@ class AutodetectorTests(TestCase):
         after = self.make_project_state([self.author_with_publisher, self.publisher])
         autodetector = MigrationAutodetector(before, after)
         changes = autodetector._detect_changes()
-        # Right number of migrations?
-        self.assertEqual(len(changes['testapp']), 1)
-        # Right number of actions?
-        migration = changes['testapp'][0]
-        self.assertEqual(len(migration.operations), 3)
-        # Right actions?
-        action = migration.operations[0]
-        self.assertEqual(action.__class__.__name__, "CreateModel")
-        self.assertEqual(action.name, "Publisher")
-        action = migration.operations[1]
-        self.assertEqual(action.__class__.__name__, "AddField")
-        self.assertEqual(action.name, "publisher")
-        action = migration.operations[2]
-        self.assertEqual(action.__class__.__name__, "RemoveField")
-        self.assertEqual(action.name, "publisher_name")
+        # Right result?
+        self.assertNumberMigrations(changes, 'testapp', 1)
+        self.assertOperationTypes(changes, 'testapp', 0, ["CreateModel", "RemoveField", "AddField"])
+        self.assertOperationAttributes(changes, 'testapp', 0, 0, name="Publisher")
+        self.assertOperationAttributes(changes, 'testapp', 0, 1, name="publisher_name")
+        self.assertOperationAttributes(changes, 'testapp', 0, 2, name="publisher")
 
     def test_foreign_key_removed_before_target_model(self):
         """