Fixed #22035 -- reordered migration operations

Now AddField actions appear in operations list before AlterUniqueTogether
actions.

Thanks to SmileyChris for the report.

django/db/migrations/autodetector.py

@@ -169,6 +169,7 @@ class MigrationAutodetector(object):
         kept_models = set(old_model_keys).intersection(new_model_keys)
         old_fields = set()
         new_fields = set()
+        unique_together_operations = []
         for app_label, model_name in kept_models:
             old_model_state = self.from_state.models[app_label, model_name]
             new_model_state = self.to_state.models[app_label, model_name]
@@ -176,15 +177,16 @@ class MigrationAutodetector(object):
             # always come before AlterFields even on separate models)
             old_fields.update((app_label, model_name, x) for x, y in old_model_state.fields)
             new_fields.update((app_label, model_name, x) for x, y in new_model_state.fields)
-            # Unique_together changes
+            # Unique_together changes. Operations will be added to migration a
+            # bit later, after fields creation. See ticket #22035.
             if old_model_state.options.get("unique_together", set()) != new_model_state.options.get("unique_together", set()):
-                self.add_to_migration(
+                unique_together_operations.append((
                     app_label,
                     operations.AlterUniqueTogether(
                         name=model_name,
                         unique_together=new_model_state.options.get("unique_together", set()),
                     )
-                )
+                ))
         # New fields
         for app_label, model_name, field_name in new_fields - old_fields:
             old_model_state = self.from_state.models[app_label, model_name]
@@ -263,6 +265,8 @@ class MigrationAutodetector(object):
                         field=new_model_state.get_field_by_name(field_name),
                     )
                 )
+        for app_label, operation in unique_together_operations:
+            self.add_to_migration(app_label, operation)
         # Alright, now add internal dependencies
         for app_label, migrations in self.migrations.items():
             for m1, m2 in zip(migrations, migrations[1:]):

tests/migrations/test_autodetector.py

@@ -29,6 +29,7 @@ class AutodetectorTests(TestCase):
     book = ModelState("otherapp", "Book", [("id", models.AutoField(primary_key=True)), ("author", models.ForeignKey("testapp.Author")), ("title", models.CharField(max_length=200))])
     book_unique = ModelState("otherapp", "Book", [("id", models.AutoField(primary_key=True)), ("author", models.ForeignKey("testapp.Author")), ("title", models.CharField(max_length=200))], {"unique_together": [("author", "title")]})
     book_unique_2 = ModelState("otherapp", "Book", [("id", models.AutoField(primary_key=True)), ("author", models.ForeignKey("testapp.Author")), ("title", models.CharField(max_length=200))], {"unique_together": [("title", "author")]})
+    book_unique_3 = ModelState("otherapp", "Book", [("id", models.AutoField(primary_key=True)), ("newfield", models.IntegerField()), ("author", models.ForeignKey("testapp.Author")), ("title", models.CharField(max_length=200))], {"unique_together": [("title", "newfield")]})
     edition = ModelState("thirdapp", "Edition", [("id", models.AutoField(primary_key=True)), ("book", models.ForeignKey("otherapp.Book"))])
     custom_user = ModelState("thirdapp", "CustomUser", [("id", models.AutoField(primary_key=True)), ("username", models.CharField(max_length=255))])
 
@@ -333,6 +334,24 @@ class AutodetectorTests(TestCase):
         self.assertEqual(action.name, "book")
         self.assertEqual(action.unique_together, set([("title", "author")]))
 
+    def test_add_field_and_unique_together(self):
+        "Tests that added fields will be created before using them in unique together"
+        before = self.make_project_state([self.author_empty, self.book])
+        after = self.make_project_state([self.author_empty, self.book_unique_3])
+        autodetector = MigrationAutodetector(before, after)
+        changes = autodetector._detect_changes()
+        # Right number of migrations?
+        self.assertEqual(len(changes['otherapp']), 1)
+        # Right number of actions?
+        migration = changes['otherapp'][0]
+        self.assertEqual(len(migration.operations), 2)
+        # Right actions order?
+        action1 = migration.operations[0]
+        action2 = migration.operations[1]
+        self.assertEqual(action1.__class__.__name__, "AddField")
+        self.assertEqual(action2.__class__.__name__, "AlterUniqueTogether")
+        self.assertEqual(action2.unique_together, set([("title", "newfield")]))
+
     def test_proxy_ignorance(self):
         "Tests that the autodetector correctly ignores proxy models"
         # First, we test adding a proxy model