Documentation and testing for F() assignment to model attributes

[Zachary Voase]

Add explicit documentation and regression tests for the use of django.db.models.F()-expressions when assigning directly to attributes on model instances (as opposed to just using them in batch update() calls).

The pattern goes something like this:

>>> from django.db.models import F
>>> from myapp.models import MyModel
>>> obj = MyModel(count=1)
>>> obj.save()
>>> obj.count
1
>>> obj.count = F('count') + 2
>>> obj.save()
>>> obj = MyModel.objects.get(pk=obj.pk) # Necessary to reload here, as the `count` attribute will still be an Expression object.
>>> obj.count
3

This should not work on records which do not already exist, since F()-expressions only work in UPDATE queries.

It's also important for this to work with any pre-save and post-save signal handlers defined in any of the django.contrib apps.

[Russell Keith-Magee]

Before anyone gets too enthusiastic in triaging this off the milestone - This came up on django-dev, and I asked Zachary to post a ticket for v1.1, since it's docs and tests for a v1.1 feature. We can easily defer this if we don't get a patch before v1.1 final, but in the meantime, this is a placeholder.

[Zachary Voase]

Initial patch. Tests pass on SQLite3.

[Zachary Voase]

I've tried to use my initiative in figuring out where the documentation should be. The tests may not pass on other DBMSes, but I don't have an installation of MySQL, Oracle or PostgreSQL on which to run them.

If you want to run the tests, just run python tests/runtests.py --settings=project.settings expressions from the root of the SVN repository.

[Russell Keith-Magee]

Interesting... your unit test has highlighted a bug - you should be able to assign foreign key values. This isn't a critical bug, and it's something we can add later without difficulty; I've opened ticket #11541 so that the issue isn't forgotten.

[Russell Keith-Magee]

(In [11322]) Fixed #11527 -- Added unit tests and documentation for the use of F() expressions in single object updates. Thanks to Zachary Voase for the patch.

[Jacob]

Milestone 1.1 deleted