Release notes missing information about change in duplicate pk behavior in 2.1

[Richard Ebeling]

We migrated a model from a AutoField as id (default behaviour) to a UUIDField in the past, using a migration that looked like this with django 2.0:
​https://gist.github.com/smcoll/8bb867dc631433c01fd0

There was a bug in SQLite though, which as of today only has the workaround listed in the bug tracker: #28541

Thus, we ended up writing our migration like this: ​https://github.com/fsr-itse/EvaP/blob/master/evap/evaluation/migrations/0062_replace_textanswer_id_with_uuid.py

When we try updating to django 2.1 now, this migration fails whenever you call migrate with the error
django.db.utils.ProgrammingError: multiple primary keys for table "evaluation_textanswer" are not allowed

This commit changed the behavior: 02365d3f38a64a5c2f3e932f23925a381d5bb151

It works if we add a RemoveField instruction for the old id before setting primary_key=True on the new UUIDField. But this will trigger the SQLite bug listed above. Changing the old AutoField to primary_key=False where the RemoveField instruction would be doesn't fix the error.

The documentation states that ​an object may not have more than one primary key.

Still, in the release notes for django 2.1 it should be noted that django's behavior was changed here. Also, I would appreciate if there was some kind of documentation on how you should migrate primary keys from a field to another if no two primary keys are allowed in between. There should be a way without squashing the migrations as we want to keep the history.

We are running postgres version 9.6.9 and psycopg2-binary version 2.7.5.
The migration error will not occur when using a SQLite database.

[Tim Graham]

It's unclear if the behavior change was intentional. Can you provide a sample project and ​bisect to find the commit where the behavior changed?

[Richard Ebeling]

This commit changed the behavior: 02365d3f38a64a5c2f3e932f23925a381d5bb151

As a sample project I can link the project we are currently working on, ​https://github.com/fsr-itse/EvaP/tree/release. A simple ./manage.py reset_db --noinput && ./manage.py migrate will trigger the error. Is this what you wanted?

[Tim Graham]

Perhaps I've made a mistake, but I tried the sample project and don't see the crash when running migrations. I tried the "master" and "release" branches and modified the DATABASES setting to use SQLite. A more minimal project to reproduce the problem would be nice.

[Richard Ebeling]

I can not reproduce this bug with SQLite either, sorry for not testing that before. We are running postgres version 9.6.9 and use psycopg2-binary version 2.7.5.. I added this information to the initial post.