Drop FK constraint on media_obj in MediaDecisionThrough, update backfillmoderationdecision command

[stacimc]

Fixes

Fixes #4512 by @krysal

Description

The backfillmoderationdecision management command creates backfilled ModerationDecision objects for existing MediaReports that have had some action taken on them (ie, not pending). The command was failing when encountering images that were deleted as part of a deindexed report, because the image no longer exists in the image table.

This PR resolves this by:

• Dropping the foreign key constraint on media_obj in the AudioDecisionThrough and ImageDecisionThrough tables. A decision can still reference the record it acted upon by its identifier, even if that record no longer exists in the primary media table.
• Updating the backfillmoderationdecision mgmt command to both access and set the plain value of the image identifier when creating the ImageDecisionThrough records, rather than attempting to use the image instance (which fails for deleted images).

Testing Instructions

Run just api/recreate.

Go to http://localhost:50280/admin/api/imagereport/ and create several new image reports:

• One with status deindexed
• One with status no_action
• One with status mature_filtered
• One with status pending

There is an issue at the moment where DeletedMedia & SensitiveMedia objects won't be automatically created for these reports. I manually made them by running just api/ipython and:

from api.models.image import Image, DeletedImage, SensitiveImage

# Using the `identifier` I selected for the image report with status deindexed
DeletedImage.objects.create(media_obj=Image.objects.get(identifier='2e22c7e7-58b7-486c-9df8-16e84c27478f')) 

# Using the `identifier` I selected for the image report with status mature_filtered
SensitiveImage.objects.create(media_obj=Image.objects.get(identifier='c296b69b-c8d7-4fc3-b820-a83de6138e0e'))

Verify in the admin that you can see the newly created DeletedImage/SensitiveImage object.

Now run the management command: just api/dj backfillmoderationdecision --no-dry-run --media-type image --moderator deploy

You should see 3 moderation decisions were created. Check them out in the admin and make sure they look as expected, and that the reports/images/decisions are all linked together appropriately. The image that was deindexed will appear empty in the admin, since the image does not exist:

However if you run just api/pgcli and actually inspect the ImageDecisionThrough table, you can see that it is set appropriately:

deploy@db:openledger> select * from api_imagedecisionthrough;
+----+-------------+--------------------------------------+
| id | decision_id | identifier                           |
|----+-------------+--------------------------------------|
| 1  | 1           | 2e22c7e7-58b7-486c-9df8-16e84c27478f |
| 2  | 2           | c296b69b-c8d7-4fc3-b820-a83de6138e0e |
| 3  | 3           | ba711b67-2cbe-487a-9feb-19d70accf912 |
+----+-------------+--------------------------------------+

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[sarayourfriend]

LGTM 👍 This fix makes sense... and actually gets rid of an unnecessary query too (passing report.media_obj to the query set would load the media object in first, I think?)

[AetherUnbound]

Thanks for the very thorough testing instructions! This worked as expected locally, I'm glad the set of changes was so simple 🚀

[dhruvkb]

This PR overlapped with mine (PR #4544), so I requested changes where we differed.

[github-actions]

This PR has migrations. Please rebase it before merging to ensure that conflicting migrations are not introduced.

[stacimc]

Agreed. That shouldn't be necessary for the backfill but I can go ahead and squash it into this migration 👍