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Revert "Fixed #16865 -- Made get_or_create use read database for init…

…ial 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 901af86.
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commit 4e9a74b81df1c7aaea2f90a3a4911920e134b275 1 parent 901af86
Carl Meyer carljm authored
2  django/db/models/query.py
View
@@ -455,9 +455,9 @@ def get_or_create(self, **kwargs):
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)
9 docs/releases/1.5.txt
View
@@ -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
==========================
24 tests/modeltests/get_or_create/tests.py
View
@@ -64,27 +64,3 @@ def test_get_or_create(self):
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)
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