Fixed #19816: pre-evaluate queryset on m2m set

In ReverseManyRelatedObjectsDescriptor.__set__, evaluate possible
queryset to avoid problems when clear() would touch data this queryset
returns.

django/db/models/fields/related.py

@@ -904,6 +904,9 @@ class ReverseManyRelatedObjectsDescriptor(object):
             raise AttributeError("Cannot set values on a ManyToManyField which specifies an intermediary model.  Use %s.%s's Manager instead." % (opts.app_label, opts.object_name))
 
         manager = self.__get__(instance)
+        # clear() can change expected output of 'value' queryset, we force evaluation
+        # of queryset before clear; ticket #19816
+        value = tuple(value)
         manager.clear()
         manager.add(*value)
 

tests/regressiontests/m2m_regress/tests.py

@@ -5,7 +5,7 @@ from django.test import TestCase
 from django.utils import six
 
 from .models import (SelfRefer, Tag, TagCollection, Entry, SelfReferChild,
-    SelfReferChildSibling, Worksheet, RegressionModelSplit)
+    SelfReferChildSibling, Worksheet, RegressionModelSplit, Line)
 
 
 class M2MRegressionTests(TestCase):
@@ -96,3 +96,16 @@ class M2MRegressionTests(TestCase):
         # causes a TypeError in add_lazy_relation
         m1 = RegressionModelSplit(name='1')
         m1.save()
+
+    def test_m2m_filter(self):
+        worksheet = Worksheet.objects.create(id=1)
+        line_hi = Line.objects.create(name="hi")
+        line_bye = Line.objects.create(name="bye")
+
+        worksheet.lines = [line_hi, line_bye]
+        hi = worksheet.lines.filter(name="hi")
+
+        worksheet.lines = hi
+        self.assertEquals(1, worksheet.lines.count())
+        self.assertEquals(1, hi.count())
+