Commit 4e9a74b8 authored by Carl Meyer's avatar Carl Meyer
Browse files

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

Thanks to Jeremy Dunck for pointing out the problem with this change. If in a
single transaction, the master deletes a record and then get_or_creates a
similar record, under the new behavior the get_or_create would find the record
in the slave db and fail to re-create it, leaving the record nonexistent, which
violates the contract of get_or_create that the record should always exist
afterwards. We need to do everything against the master here in order to ensure
correctness.

This reverts commit 901af865.
parent 901af865

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

+0 −9
Original line number Diff line number Diff line
@@ -338,15 +338,6 @@ 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

+0 −24
Original line number Diff line number Diff line
@@ -64,27 +64,3 @@ 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)