Fixed #4102 -- Allow update of specific fields in model.save()

Added the ability to update only part of the model's fields in
model.save() by introducing a new kwarg "update_fields". Thanks
to all the numerous reviewers and commenters in the ticket

tests/modeltests/update_only_fields/tests.py

@@ -0,0 +1,146 @@
+from __future__ import absolute_import
+
+from django.test import TestCase
+from django.db.models.signals import pre_save, post_save
+from .models import Person, Employee, ProxyEmployee, Profile, Account
+
+
+class UpdateOnlyFieldsTests(TestCase):
+    def test_update_fields_basic(self):
+        s = Person.objects.create(name='Sara', gender='F')
+        self.assertEqual(s.gender, 'F')
+
+        s.gender = 'M'
+        s.name = 'Ian'
+        s.save(update_fields=['name'])
+
+        s = Person.objects.get(pk=s.pk)
+        self.assertEqual(s.gender, 'F')
+        self.assertEqual(s.name, 'Ian')
+
+    def test_update_fields_m2m(self):
+        profile_boss = Profile.objects.create(name='Boss', salary=3000)
+        e1 = Employee.objects.create(name='Sara', gender='F',
+            employee_num=1, profile=profile_boss)
+
+        a1 = Account.objects.create(num=1)
+        a2 = Account.objects.create(num=2)
+
+        e1.accounts = [a1,a2]
+
+        with self.assertRaises(ValueError):
+            e1.save(update_fields=['accounts'])
+
+    def test_update_fields_inheritance(self):
+        profile_boss = Profile.objects.create(name='Boss', salary=3000)
+        profile_receptionist = Profile.objects.create(name='Receptionist', salary=1000)
+
+        e1 = Employee.objects.create(name='Sara', gender='F',
+            employee_num=1, profile=profile_boss)
+
+        e1.name = 'Ian'
+        e1.gender = 'M'
+        e1.save(update_fields=['name'])
+
+        e2 = Employee.objects.get(pk=e1.pk)
+        self.assertEqual(e2.name, 'Ian')
+        self.assertEqual(e2.gender, 'F')
+        self.assertEqual(e2.profile, profile_boss)
+
+        e2.profile = profile_receptionist
+        e2.name = 'Sara'
+        e2.save(update_fields=['profile'])
+
+        e3 = Employee.objects.get(pk=e1.pk)
+        self.assertEqual(e3.name, 'Ian')
+        self.assertEqual(e3.profile, profile_receptionist)
+
+    def test_update_fields_inheritance_with_proxy_model(self):
+        profile_boss = Profile.objects.create(name='Boss', salary=3000)
+        profile_receptionist = Profile.objects.create(name='Receptionist', salary=1000)
+
+        e1 = ProxyEmployee.objects.create(name='Sara', gender='F',
+            employee_num=1, profile=profile_boss)
+
+        e1.name = 'Ian'
+        e1.gender = 'M'
+        e1.save(update_fields=['name'])
+
+        e2 = ProxyEmployee.objects.get(pk=e1.pk)
+        self.assertEqual(e2.name, 'Ian')
+        self.assertEqual(e2.gender, 'F')
+        self.assertEqual(e2.profile, profile_boss)
+
+        e2.profile = profile_receptionist
+        e2.name = 'Sara'
+        e2.save(update_fields=['profile'])
+
+        e3 = ProxyEmployee.objects.get(pk=e1.pk)
+        self.assertEqual(e3.name, 'Ian')
+        self.assertEqual(e3.profile, profile_receptionist)
+
+    def test_update_fields_signals(self):
+        p = Person.objects.create(name='Sara', gender='F')
+        pre_save_data = []
+        def pre_save_receiver(**kwargs):
+            pre_save_data.append(kwargs['update_fields'])
+        pre_save.connect(pre_save_receiver)
+        post_save_data = []
+        def post_save_receiver(**kwargs):
+            post_save_data.append(kwargs['update_fields'])
+        post_save.connect(post_save_receiver)
+        p.save(update_fields=['name'])
+        self.assertEqual(len(pre_save_data), 1)
+        self.assertEqual(len(pre_save_data[0]), 1)
+        self.assertTrue('name' in pre_save_data[0])
+        self.assertEqual(len(post_save_data), 1)
+        self.assertEqual(len(post_save_data[0]), 1)
+        self.assertTrue('name' in post_save_data[0])
+
+    def test_update_fields_incorrect_params(self):
+        s = Person.objects.create(name='Sara', gender='F')
+
+        with self.assertRaises(ValueError):
+            s.save(update_fields=['first_name'])
+
+        with self.assertRaises(ValueError):
+            s.save(update_fields="name")
+
+    def test_empty_update_fields(self):
+        s = Person.objects.create(name='Sara', gender='F')
+        pre_save_data = []
+        def pre_save_receiver(**kwargs):
+            pre_save_data.append(kwargs['update_fields'])
+        pre_save.connect(pre_save_receiver)
+        post_save_data = []
+        def post_save_receiver(**kwargs):
+            post_save_data.append(kwargs['update_fields'])
+        post_save.connect(post_save_receiver)
+        # Save is skipped.
+        with self.assertNumQueries(0):
+            s.save(update_fields=[])
+        # Signals were skipped, too...
+        self.assertEqual(len(pre_save_data), 0)
+        self.assertEqual(len(post_save_data), 0)
+
+    def test_num_queries_inheritance(self):
+        s = Employee.objects.create(name='Sara', gender='F')
+        s.employee_num = 1
+        s.name = 'Emily'
+        with self.assertNumQueries(1):
+            s.save(update_fields=['employee_num'])
+        s = Employee.objects.get(pk=s.pk)
+        self.assertEqual(s.employee_num, 1)
+        self.assertEqual(s.name, 'Sara')
+        s.employee_num = 2
+        s.name = 'Emily'
+        with self.assertNumQueries(1):
+            s.save(update_fields=['name'])
+        s = Employee.objects.get(pk=s.pk)
+        self.assertEqual(s.name, 'Emily')
+        self.assertEqual(s.employee_num, 1)
+        # A little sanity check that we actually did updates...
+        self.assertEqual(Employee.objects.count(), 1)
+        self.assertEqual(Person.objects.count(), 1)
+        with self.assertNumQueries(2):
+            s.save(update_fields=['name', 'employee_num'])