values() after extra() gives incorrect error message

[aimestereo]

In documentation there's comment in values() section about some behavior of extra

​https://docs.djangoproject.com/en/1.9/ref/models/querysets/#django.db.models.query.QuerySet.values

If you use a values() clause after an extra() call, any fields defined by a select argument in the extra() must be explicitly included in the values() call. Any extra() call made after a values() call will have its extra selected fields ignored.

It's points that any extra fields after values() call will be ignored. But I think that it's not correct and need more description.

For example it could be not obvious that next simple construction will cause exception and such an not obvious one!

User.objects.values('username').extra(select={'test_field': 1}).values('test_field', 'username')

FieldError: Cannot resolve keyword 'test_field' into field. Choices are: date_joined, email, first_name, groups, id, is_active, is_staff, is_superuser, last_login, last_name, logentry, password, test_field, user_permissions, username

I want to point out that test_field is listed in choices that's very confusing. It happens because of all checks is runs against extra_select but error is uses extra as part of available choices.

sorted(list(get_field_names_from_opts(opts)) + list(self.extra)
                               + list(self.annotation_select))

I can help you with this small change.
​https://github.com/django/django/pull/6225

[Tim Graham]

Note ​the documentation, "This is an old API that we aim to deprecate at some point in the future. Use it only if you cannot express your query using other queryset methods. If you do need to use it, please file a ticket using the QuerySet.extra keyword with your use case (please check the list of existing tickets first) so that we can enhance the QuerySet API to allow removing extra(). We are no longer improving or fixing bugs for this method."

In light of that, is it worth fixing this?

[Tim Graham]

I guess the patch is simple enough. It just needs a test.

[Tim Graham]

The patch causes a regression in another unit test, so I'm not going to bother spending more time on this.