Commit 901af865 authored by Carl Meyer's avatar Carl Meyer
Browse files

Fixed #16865 -- Made get_or_create use read database for initial get query. 

Thanks Rick van Hattem for the report and trbs for the patch.
parent 44767f2c

django/db/models/query.py

+1 −1
Original line number Diff line number Diff line
@@ -455,9 +455,9 @@ class QuerySet(object): 
            if f.attname in lookup:

                lookup[f.name] = lookup.pop(f.attname)

        try:

            self._for_write = True

            return self.get(**lookup), False

        except self.model.DoesNotExist:

            self._for_write = True

            try:

                params = dict([(k, v) for k, v in kwargs.items() if '__' not in k])

                params.update(defaults)

docs/releases/1.5.txt

+9 −0
Original line number Diff line number Diff line
@@ -338,6 +338,15 @@ Miscellaneous 
  needs. The new default value is `0666` (octal) and the current umask value

  is first masked out.



* In a multi-database situation, ``get_or_create()`` will now use a read

  database for the initial ``get`` attempt (previously, it used only the write

  database for all queries). This change reduces load on the write (master)

  database, in exchange for slightly more frequent false-negatives on the

  initial ``get`` due to replication lag. In those cases the subsequent insert

  will still go to the master and fail, after which the existing object will be

  fetched from the master.





Features deprecated in 1.5

==========================

tests/modeltests/get_or_create/tests.py

+24 −0
Original line number Diff line number Diff line
@@ -64,3 +64,27 @@ class GetOrCreateTests(TestCase): 
            formatted_traceback = traceback.format_exc()

            self.assertIn('obj.save', formatted_traceback)





    def test_initial_get_on_read_db(self):

        """

        get_or_create should only set _for_write when it's actually doing a

        create action. This makes sure that the initial .get() will be able to

        use a slave database. Specially when some form of database pinning is

        in place this will help to not put all the SELECT queries on the

        master. Refs #16865.



        """

        qs = Person.objects.get_query_set()

        p, created = qs.get_or_create(

            first_name="Stuart", last_name="Sutcliffe", defaults={

                "birthday": date(1940, 6, 23),

                }

            )

        self.assertTrue(created)

        self.assertTrue(qs._for_write)



        qs = Person.objects.get_query_set()

        p, created = qs.get_or_create(

            first_name="Stuart", last_name="Sutcliffe")

        self.assertFalse(created)

        self.assertFalse(qs._for_write)