#30022: Doc how to combine post_save signal with on_commit to alter a m2m
relation
when saving a model instance
-------------------------------------+-------------------------------------
Reporter: George Tantiras | Owner: nobody
Type: | Status: closed
Cleanup/optimization |
Component: Documentation | Version: 2.1
Severity: Normal | Resolution: wontfix
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Changes (by Simon Charette):
* status: new => closed
* resolution: => wontfix
Comment:
Thank you for the suggestion but it feels like a pretty specific example
to document as this is not something you want to do in all cases.
If I understand where you are coming from you want to make sure changes
performed in `save()` are committed to the database in the same
transaction as the m2m alterations.
In your "group" case your suggested approach will work fine in cases where
`save()` in not called in within a `transaction.atomic()` block will have
surprising results if this doesn't hold true.
For example in
{{{#!python
@transaction.atomic
def accept_group_invite(request, group_id):
validate_and_add_to_group(request.user, group_id)
# The below line would always fail in your case because the on_commit
# receiver wouldn't be called until exiting this function.
if request.user.has_perm('group_permission'):
do_something()
...
}}}
`do_something()` would never be called because
`validate_and_add_to_group()`'s `save()` calls that would trigger the
`post_save` receiver would only queue the group additions to be performed
`on_commit` which wouldn't happen until the `accept_group_invite` function
exits.
If you want to make sure both `save()` and its `pre_save` and `post_save`
side effects are performed in as an atomic operation (either succeed or
fail) then you should simply wrap your `save()` calls in an `atomic`
block.
--
Ticket URL: <https://code.djangoproject.com/ticket/30022#comment:1>
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