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

docs/ref/models/instances.txt

@@ -135,7 +135,7 @@ Saving objects
 
 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, update_fields=None])
 
 .. versionadded:: 1.2
    The ``using`` argument was added.
@@ -289,6 +289,8 @@ almost always do the right thing and trying to override that will lead to
 errors that are difficult to track down. This feature is for advanced use
 only.
 
+Using ``update_fields`` will force an update similarly to ``force_update``.
+
 Updating attributes based on existing fields
 --------------------------------------------
 
@@ -334,6 +336,26 @@ For more details, see the documentation on :ref:`F() expressions
 <query-expressions>` and their :ref:`use in update queries
 <topics-db-queries-update>`.
 
+Specifying which fields to save
+-------------------------------
+
+.. versionadded:: 1.5
+
+If ``save()`` is passed a list of field names in keyword argument
+``update_fields``, only the fields named in that list will be updated.
+This may be desirable if you want to update just one or a few fields on
+an object. 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(update_fields=['name'])
+
+The ``update_fields`` argument can be any iterable containing strings. An
+empty ``update_fields`` iterable will skip the save. A value of None will
+perform an update on all fields.
+
+Specifying ``update_fields`` will force an update.
+
 Deleting objects
 ================