Index: django/db/models/base.py
===================================================================
--- django/db/models/base.py	(revision 5992)
+++ django/db/models/base.py	(working copy)
@@ -19,6 +19,11 @@
 import sys
 import os
 
+try:
+    set
+except NameError:
+    from sets import Set as set   # Python 2.3 fallback
+
 class ModelBase(type):
     "Metaclass for all models"
     def __new__(cls, name, bases, attrs):
@@ -97,13 +102,29 @@
     def __ne__(self, other):
         return not self.__eq__(other)
 
+    def __setattr__(self, name, value):
+        if name != '_modified_attrs' and (not hasattr(self, name) or
+                                          value != getattr(self, name)):
+            if hasattr(self, '_modified_attrs'):
+                if name not in self._modified_attrs: 
+                    self._modified_attrs.add(name)
+            else:
+                self._modified_attrs = set((name,))
+        super(Model, self).__setattr__(name, value)
+
+    def _reset_modified_attrs(self):
+        try:
+            self._modified_attrs.clear()
+        except AttributeError:
+            pass
+
     def __init__(self, *args, **kwargs):
         dispatcher.send(signal=signals.pre_init, sender=self.__class__, args=args, kwargs=kwargs)
 
         # There is a rather weird disparity here; if kwargs, it's set, then args
-        # overrides it. It should be one or the other; don't duplicate the work
+        # overrides it. It should be one or the other; don't duplicate the work.
         # The reason for the kwargs check is that standard iterator passes in by
-        # args, and nstantiation for iteration is 33% faster.
+        # args, and instantiation for iteration is 33% faster.
         args_len = len(args)
         if args_len > len(self._meta.fields):
             # Daft, but matches old exception sans the err msg.
@@ -202,9 +223,14 @@
     _prepare = classmethod(_prepare)
 
     def save(self, raw=False):
+        "Save model object to database, creating if doesn't exist, otherwise updating"
         dispatcher.send(signal=signals.pre_save, sender=self.__class__, instance=self)
 
         non_pks = [f for f in self._meta.fields if not f.primary_key]
+        if hasattr(self, '_modified_attrs'):
+            ma = self._modified_attrs
+            non_pks = [f for f in non_pks if (f.name in ma or f.attname in ma)]
+            self._reset_modified_attrs()
         cursor = connection.cursor()
 
         qn = connection.ops.quote_name
Index: django/db/models/query.py
===================================================================
--- django/db/models/query.py	(revision 5992)
+++ django/db/models/query.py	(working copy)
@@ -204,6 +204,10 @@
                                                     index_start=0, max_depth=self._max_related_depth)
                 else:
                     obj = self.model(*row[:index_end])
+                    # Models keep a track of modified attrs to choose which
+                    # fields to save. Since we're just pulling from the
+                    # database, nothing has changed yet.
+                    obj._reset_modified_attrs()
                 for i, k in enumerate(extra_select):
                     setattr(obj, k[0], row[index_end+i])
                 yield obj
Index: tests/modeltests/update_fields/__init__.py
===================================================================
Index: tests/modeltests/update_fields/models.py
===================================================================
--- tests/modeltests/update_fields/models.py	(revision 0)
+++ tests/modeltests/update_fields/models.py	(revision 0)
@@ -0,0 +1,164 @@
+"""
+#. Save only modified fields on update
+"""
+from django.db import models
+
+class Article(models.Model):
+    headline = models.CharField(maxlength=100, default='Default headline')
+    body = models.TextField()
+    pub_date = models.DateTimeField()
+
+    class Meta:
+        ordering = ('pub_date','headline')
+
+    def __str__(self):
+        return self.headline
+
+__test__ = {'API_TESTS':"""
+# Create a couple of Articles.
+>>> from datetime import datetime
+>>> a = Article(headline='Article 1', body='Body 1', pub_date=datetime(2007, 5, 5))
+>>> a.save()
+
+# Change headline and body on different instances, then save both to verify
+# that each only updated the modified field.
+>>> a1 = Article.objects.get(pk=a.id)
+>>> a2 = Article.objects.get(pk=a.id)
+>>> a1.headline = 'Changed article 1'
+>>> a2.body = 'Changed body 1'
+>>> a1.save()
+>>> a2.save()
+>>> a3 = Article.objects.get(pk=a.id)
+>>> a3.headline
+u'Changed article 1'
+>>> a3.body
+u'Changed body 1'
+
+# Fields entered at istantiation of a model which will already exists should be
+# saved as well.
+>>> a = Article(id=a.id, headline='Reset article 1', body='Reset body 1', pub_date=datetime(2007, 5, 5))
+>>> a.save()
+>>> a3 = Article.objects.get(pk=a.id)
+>>> a3.headline
+u'Reset article 1'
+>>> a3.body
+u'Reset body 1'
+"""}
+"""
+#. Save only modified fields on update
+"""
+from django.db import models
+
+class Article(models.Model):
+    headline = models.CharField(maxlength=100, default='Default headline')
+    body = models.TextField()
+    pub_date = models.DateTimeField()
+
+    class Meta:
+        ordering = ('pub_date','headline')
+
+    def __str__(self):
+        return self.headline
+
+__test__ = {'API_TESTS':"""
+# Create a couple of Articles.
+>>> from datetime import datetime
+>>> a = Article(headline='Article 1', body='Body 1', pub_date=datetime(2007, 5, 5))
+>>> a.save()
+
+# Change headline and body on different instances, then save both to verify
+# that each only updated the modified field
+>>> a1 = Article.objects.get(pk=a.id)
+>>> a2 = Article.objects.get(pk=a.id)
+>>> a1.headline = 'Changed article 1'
+>>> a2.body = 'Changed body 1'
+>>> a1.save()
+>>> a2.save()
+>>> a3 = Article.objects.get(pk=a.id)
+>>> a3.headline
+'Changed article 1'
+>>> a3.body
+'Changed body 1'
+"""}
+"""
+#. Save only modified fields on update
+"""
+from django.db import models
+
+class Article(models.Model):
+    headline = models.CharField(maxlength=100, default='Default headline')
+    body = models.TextField()
+    pub_date = models.DateTimeField()
+
+    class Meta:
+        ordering = ('pub_date','headline')
+
+    def __str__(self):
+        return self.headline
+
+__test__ = {'API_TESTS':"""
+# Create a couple of Articles.
+>>> from datetime import datetime
+>>> a = Article(headline='Article 1', body='Body 1', pub_date=datetime(2007, 5, 5))
+>>> a.save()
+
+# Change headline and body on different instances, then save both to verify
+# that each only updated the modified field.
+>>> a1 = Article.objects.get(pk=a.id)
+>>> a2 = Article.objects.get(pk=a.id)
+>>> a1.headline = 'Changed article 1'
+>>> a2.body = 'Changed body 1'
+>>> a1.save()
+>>> a2.save()
+>>> a3 = Article.objects.get(pk=a.id)
+>>> a3.headline
+u'Changed article 1'
+>>> a3.body
+u'Changed body 1'
+
+# Fields entered at istantiation of a model which will already exists should be
+# saved as well.
+>>> a = Article(id=a.id, headline='Reset article 1', body='Reset body 1', pub_date=datetime(2007, 5, 5))
+>>> a.save()
+>>> a3 = Article.objects.get(pk=a.id)
+>>> a3.headline
+u'Reset article 1'
+>>> a3.body
+u'Reset body 1'
+"""}
+"""
+#. Save only modified fields on update
+"""
+from django.db import models
+
+class Article(models.Model):
+    headline = models.CharField(maxlength=100, default='Default headline')
+    body = models.TextField()
+    pub_date = models.DateTimeField()
+
+    class Meta:
+        ordering = ('pub_date','headline')
+
+    def __str__(self):
+        return self.headline
+
+__test__ = {'API_TESTS':"""
+# Create a couple of Articles.
+>>> from datetime import datetime
+>>> a = Article(headline='Article 1', body='Body 1', pub_date=datetime(2007, 5, 5))
+>>> a.save()
+
+# Change headline and body on different instances, then save both to verify
+# that each only updated the modified field
+>>> a1 = Article.objects.get(pk=a.id)
+>>> a2 = Article.objects.get(pk=a.id)
+>>> a1.headline = 'Changed article 1'
+>>> a2.body = 'Changed body 1'
+>>> a1.save()
+>>> a2.save()
+>>> a3 = Article.objects.get(pk=a.id)
+>>> a3.headline
+'Changed article 1'
+>>> a3.body
+'Changed body 1'
+"""}