Introduce a generic update service

Author: wencakisa

styleguide_example/common/services.py

@@ -0,0 +1,39 @@
+from typing import TypeVar
+
+from django.db import transaction
+
+T = TypeVar('T')
+
+
+@transaction.atomic
+def generic_update(*, instance: T, **fields_to_update) -> T:
+    """
+    Generic update service meant to be reused in local update services
+
+    For example:
+
+    def user_update(*, user: User, **fields_to_update) -> User:
+        user = generic_update(instance=user, **fields_to_update)
+
+        // Do other actions with the user here
+
+        return user
+    """
+    # If the passed instance is `None` - do nothing.
+    if instance is None:
+        return instance
+
+    # If there's nothing to update - do not perform unnecessary actions.
+    if not fields_to_update:
+        return instance
+
+    for attr, value in fields_to_update.items():
+        setattr(instance, attr, value)
+
+    instance.full_clean()
+    # Update only the fields that are meant to be updated.
+    # Django docs reference: https://docs.djangoproject.com/en/dev/ref/models/instances/#specifying-which-fields-to-save
+    update_fields = list(fields_to_update.keys())
+    instance.save(update_fields=update_fields)
+
+    return instance

styleguide_example/common/tests/services/__init__.py

@@ -0,0 +1,45 @@
+import unittest
+from unittest.mock import Mock
+
+from styleguide_example.common.services import generic_update
+
+
+class GenericUpdateTests(unittest.TestCase):
+    def test_generic_update_returns_none_if_passed_instance_is_none(self):
+        instance = None
+        updated_instance = generic_update(instance=instance)
+
+        self.assertIsNone(updated_instance)
+
+    def test_generic_update_returns_none_if_no_update_fields_are_passed(self):
+        instance = Mock()
+        updated_instance = generic_update(instance=instance)
+
+        self.assertEqual(instance, updated_instance)
+        instance.full_clean.assert_not_called()
+        instance.save.assert_not_called()
+
+    def test_generic_update_sets_passed_fields_and_executes_full_clean_and_save(self):
+        instance = Mock(
+            field_a=None,
+            field_b=None,
+            field_c=None
+        )
+        fields_to_update = {
+            'field_a': 'value_a',
+            'field_b': 'value_b',
+        }
+
+        self.assertIsNone(instance.field_a)
+        self.assertIsNone(instance.field_b)
+        self.assertIsNone(instance.field_c)
+
+        updated_instance = generic_update(instance=instance, **fields_to_update)
+
+        instance.full_clean.assert_called_once()
+        instance.save.assert_called_once_with(update_fields=['field_a', 'field_b'])
+
+        self.assertEqual(updated_instance.field_a, 'value_a')
+        self.assertEqual(updated_instance.field_b, 'value_b')
+        # `field_c` is not updated because it is not passed as a field to update
+        self.assertIsNone(updated_instance.field_c)