Ticket #4102: only_fields_django-1.4.patch

docs/ref/models/instances.txt

@@ -135 +135 @@
 
 To save an object back to the database, call ``save()``:
 
-.. method:: Model.save([force_insert=False, force_update=False, using=DEFAULT_DB_ALIAS])
+.. method:: Model.save([force_insert=False, force_update=False, using=DEFAULT_DB_ALIAS, only_fields=None])
 
 .. versionadded:: 1.2
    The ``using`` argument was added.
@@ -334 +334 @@
 <query-expressions>` and their :ref:`use in update queries
 <topics-db-queries-update>`.
 
+Specifying which fields to save
+-------------------------------
+
+If ``save()`` is passed a list of field names as keyword argument ``only_fields``, 
+only the fields named in that list will be saved to the database. This may be 
+desirable if you want to update just one or a few fields on an object, as there 
+will be a slight performance benefit from preventing all of the model fields 
+from being updated in the database. For example:
+
+    product.name = 'Name changed again'
+    product.save(only_fields=['name'])
+
+
 Deleting objects
 ================
 

django/db/models/base.py

@@ -449 +449 @@
             return getattr(self, field_name)
         return getattr(self, field.attname)
 
-    def save(self, force_insert=False, force_update=False, using=None):
+    def save(self, force_insert=False, force_update=False, using=None, only_fields=None):
         """
         Saves the current instance. Override this in a subclass if you want to
         control the saving process.
@@ -460 +460 @@
         """
         if force_insert and force_update:
             raise ValueError("Cannot force both insert and updating in model saving.")
-        self.save_base(using=using, force_insert=force_insert, force_update=force_update)
+        self.save_base(using=using, force_insert=force_insert, 
+                        force_update=force_update, only_fields=only_fields)
 
     save.alters_data = True
 
     def save_base(self, raw=False, cls=None, origin=None, force_insert=False,
-            force_update=False, using=None):
+            force_update=False, using=None, only_fields=None):
         """
         Does the heavy-lifting involved in saving. Subclasses shouldn't need to
         override this method. It's separate from save() in order to hide the
@@ -513 +514 @@
         if not meta.proxy:
             non_pks = [f for f in meta.local_fields if not f.primary_key]
 
+            if only_fields: 
+                non_pks = [f for f in non_pks if f.name in only_fields] 
+
             # First, try an UPDATE. If that doesn't update anything, do an INSERT.
             pk_val = self._get_pk_val(meta)
             pk_set = pk_val is not None

tests/modeltests/update_only_fields/__init__.py

@@ +1 @@
+# -*- coding: utf-8 -*-
+
+

tests/modeltests/update_only_fields/tests.py

@@ +1 @@
+from __future__ import absolute_import
+
+from django.test import TestCase
+from .models import Person
+
+class UpdateOnlyFieldsTests(TestCase):
+    def test_only_update(self):
+        s = Person.objects.create(name='Sara', gender='F')
+        self.assertEqual(s.gender, 'F')
+
+        s.gender = 'M'
+        s.save(only_fields=['name'])
+
+        s = Person.objects.get(pk=s.pk)
+        self.assertEqual(s.gender, 'F')

tests/modeltests/update_only_fields/models.py

@@ +1 @@
+
+from django.db import models
+
+GENDER_CHOICES = (
+    ('M', 'Male'),
+    ('F', 'Female'),
+)
+
+class Person(models.Model):
+    name = models.CharField(max_length=20)
+    gender = models.CharField(max_length=1, choices=GENDER_CHOICES)
+
+    def __unicode__(self):
+        return self.name