id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
9205	Add savepoint protection to Model.save()	Richard Davies <richard.davies@…>	nobody	"[8314] introduced savepoints - these are a no-op for all databases except for Postgresql, on which they must be used to continue using a database connection after an IntegrityError.

IntegrityError can be raised in save() in several cases:
 * because of a conflict on a unique field (example below)
 * when using force_insert or force_update if this is not possible
 * if there is a race between when save() decides whether to insert/update and when it actually does so

At present, the user should wrap calls to save() with savepoints if they are likely to fail for any of these reasons.

This patch changes save() to always wrap the call to save_base() in savepoints, meaning that all of the above cases leave a working database connection and the user does not need to wrap. It means that ""user"" code wrapping save() can be removed from several places in Django and its test suite. The complete test suite still passes on Postgresql.

Possibly the savepoints should move _inside_ save_base(), so that direct calls also benefit from them. Doing this naively leads to extra test suite failures, which I have not investigated.

On performance: we are now making many more savepoint calls, which could have performance implications. These calls are no-ops on non-Postgresql backends, so should have little impact there. On Postgresql, time to run the complete test suite was unchanged on my development machine. So, I conclude that there are no performance implications.

--------------------------

Example of IntegrityError from unique field conflict:

{{{
I have a simple model:

================
class Test(models.Model):
  data1 = models.IntegerField(unique=True)
  data2 = models.IntegerField()
================

and a view using it:

================
def render(request):
  a = Test()
  a.data1 = 1
  a.data2 = 10
  a.save()

  b = Test()
  b.data1 = 1
  b.data2 = 20
  try:
    b.save()
  except IntegrityError:
    # Expected since data1 not unique
    # savepoint protection needed here for postgresql today
    pass

  c = Test()
  c.data1 = 2
  c.data2 = 30
  # Today: c is saved by most database backends, but not by postgresql.
  #        savepoint protection would be needed above for postgresql.
  #
  # With patch: c is saved by all database backends.
  c.save()

  return HttpResponse('Test')
}}}"	New feature	closed	Database layer (models, ORM)	dev	Normal	wontfix		richard.davies@…	Design decision needed	1	0	0	0	0	0