Fixed #19918 -- Modified select_for_update to run on the write database.

        # 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

import unittest

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 multiple_database.tests import TestRouter

from .models import Person

# Some tests require threading, which might not be available. So create a

        """

        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