Regression in reverse one-to-one field when in readonly_fields

[Steven Davidson]

When upgrading from 1.7 to 1.8(.8) I started encountering errors in the admin with a reverse one to one relationship when it was in readonly_fields. I assume this is a regression. Naturally, I am happy to help fix it (if it is not by design), given a steer!

The attached simple project shows the issue (it was created with Django 1.8.8). Python 2.7.10

cd /tmp
git clone ​https://github.com/damycra/django-readonly-onetoone.git
cd django-readonly-onetoone/
mkvirtualenv dj_onetoone

# make sure virtualenv is activated, then:
pip install -r requirements.txt
./manage.py migrate
./manage.py createsuperuser
./manage.py runserver
# open ​http://127.0.0.1:8000/admin/simple/a/add/
# create an A object
# create a B object referencing the A ​http://127.0.0.1:8000/admin/simple/b/add
# return to original A object ​http://127.0.0.1:8000/admin/simple/a/1/

# now upgrade to Django 1.8.8
pip install Django==1.8.8
./manage.py migrate
./manage.py runserver
# return to ​http://127.0.0.1:8000/admin/simple/a/1/
AttributeError at /admin/simple/a/1/
OneToOneRel object has no attribute 'rel'

# now upgrade to Django 1.9.1
pip install Django==1.9.1
./manage.py migrate
./manage.py runserver
# go to ​http://127.0.0.1:8000/admin/simple/a/1/change/
AttributeError at /admin/simple/a/1/change/
'OneToOneRel' object has no attribute 'flatchoices'

A simple workaround is to use a method on ModelAdmin

class WorkingAAdmin(admin.ModelAdmin):
    readonly_fields = ['working_b']
    def working_b(self, instance):
        return instance.b
    working_b.short_description = 'working'

See also https://code.djangoproject.com/ticket/24851#comment:8

[Steven Davidson]

Zip of Github project