[Django Upgrade] [ENG-3947] Direct assignment fix for Django 3

[Johnetordoff]

Purpose

Note: PR description added by reviewer

This incompatibility comes from Feature Removed in 2.0

Support for direct assignment to a reverse foreign key or many-to-many relation is removed.

This might have been missed by (or was added after) #9300.

Changes

Replaced direct assignment with .add() and/or .set().

QA Notes

Documentation

Side Effects

Ticket

Part of https://openscience.atlassian.net/browse/ENG-3947

[cslzchen]

@Johnetordoff Can you update the PR description to include

• A brief description of what is broken and how the fix work.
• Is there a source of this incompatibility (e.g. django docs and/or references)

My other concern is: if this is an incompatibility caused by Django upgrade, I would expect to see more places broken. Have you taken a look at other places that we may have this "direct assignment issue" (i.e. not found by unit tests)?

[Johnetordoff]

Yes I see your concerns and I myself am confused by the fact that I can't find direct mention of this change in our doc, but here is the error gist and test job.

[cslzchen]

Ah, I see. The error is TypeError: Direct assignment to the forward side of a many-to-many set is prohibited. Must be a missing-documentation incompatibility from django. (See updated comment below). Here is some context I found about this error: https://stackoverflow.com/questions/50015204/direct-assignment-to-the-forward-side-of-a-many-to-many-set-is-prohibited-use-e. And PR looks good, I will update the PR description with more context and then merge.

[cslzchen]

Updated:

The incompatibility comes from Feature Removed in 2.0

Support for direct assignment to a reverse foreign key or many-to-many relation is removed.

This might have been missed by (or was added after) #9300.