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

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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 901af865505310a70dd02ea5b3becbf45819b652.
This commit is contained in:
Carl Meyer 2012-09-19 11:15:12 -06:00
parent 901af86550
commit 4e9a74b81d
3 changed files with 1 additions and 34 deletions
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2
django/db/models/query.py
View File

@ -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)

9
docs/releases/1.5.txt
View File

@ -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 File

@ -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)