Fixed #16856 - Added a way to clear select_related.

Thanks Carl for the suggestion and David Cramer for the patch.

django/db/models/query.py

@@ -645,6 +645,8 @@ class QuerySet(object):
 
         If fields are specified, they must be ForeignKey fields and only those
         related objects are included in the selection.
+
+        If select_related(None) is called, the list is cleared.
         """
         if 'depth' in kwargs:
             warnings.warn('The "depth" keyword argument has been deprecated.\n'
@@ -654,7 +656,9 @@ class QuerySet(object):
             raise TypeError('Unexpected keyword arguments to select_related: %s'
                     % (list(kwargs),))
         obj = self._clone()
-        if fields:
+        if fields == (None,):
+            obj.query.select_related = False
+        elif fields:
             if depth:
                 raise TypeError('Cannot pass both "depth" and fields to select_related()')
             obj.query.add_select_related(fields)

docs/ref/models/querysets.txt

@@ -774,6 +774,13 @@ You can also refer to the reverse direction of a
 is defined. Instead of specifying the field name, use the :attr:`related_name
 <django.db.models.ForeignKey.related_name>` for the field on the related object.
 
+.. versionadded:: 1.6
+
+If you need to clear the list of related fields added by past calls of
+``select_related`` on a ``QuerySet``, you can pass ``None`` as a parameter::
+
+   >>> without_relations = queryset.select_related(None)
+
 .. deprecated:: 1.5
     The ``depth`` parameter to ``select_related()`` has been deprecated. You
     should replace it with the use of the ``(*fields)`` listing specific

docs/releases/1.6.txt

@@ -287,6 +287,11 @@ Minor features
   helper for testing formset errors:
   :meth:`~django.test.SimpleTestCase.assertFormsetError`.
 
+* The list of related fields added to a
+  :class:`~django.db.models.query.QuerySet` by
+  :meth:`~django.db.models.query.QuerySet.select_related` can be cleared using
+  ``select_related(None)``.
+
 Backwards incompatible changes in 1.6
 =====================================
 

tests/select_related/tests.py

@@ -172,3 +172,7 @@ class SelectRelatedTests(TestCase):
                 Species.objects.select_related,
                 'genus__family__order', depth=4
             )
+
+    def test_none_clears_list(self):
+        queryset = Species.objects.select_related('genus').select_related(None)
+        self.assertEqual(queryset.query.select_related, False)