[1.6.x] Fixed #19918 -- Modified select_for_update to run on the write database.

Backport of 1c64a0f29e from master

django/db/models/query.py

@@ -635,6 +635,7 @@ class QuerySet(object):
         # Default to false for nowait
         nowait = kwargs.pop('nowait', False)
         obj = self._clone()
+        obj._for_write = True
         obj.query.select_for_update = True
         obj.query.select_for_update_nowait = nowait
         return obj

tests/select_for_update/tests.py

@@ -4,12 +4,14 @@ import sys
 import time
 
 from django.conf import settings
-from django.db import transaction, connection
+from django.db import transaction, connection, router
 from django.db.utils import ConnectionHandler, DEFAULT_DB_ALIAS, DatabaseError
 from django.test import (TransactionTestCase, skipIfDBFeature,
     skipUnlessDBFeature)
 from django.utils import unittest
 
+from multiple_database.tests import TestRouter
+
 from .models import Person
 
 # Some tests require threading, which might not be available. So create a
@@ -268,3 +270,13 @@ class SelectForUpdateTests(TransactionTestCase):
         """
         people = list(Person.objects.select_for_update())
         self.assertTrue(transaction.is_dirty())
+
+    @skipUnlessDBFeature('has_select_for_update')
+    def test_select_for_update_on_multidb(self):
+        old_routers = router.routers
+        try:
+            router.routers = [TestRouter()]
+            query = Person.objects.select_for_update()
+            self.assertEqual(router.db_for_write(Person), query.db)
+        finally:
+            router.routers = old_routers