AO3-4693 Fix collection deletion for collections with a challenge

[ayasayadi1]

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Issue

https://otwarchive.atlassian.net/browse/AO3-4693

Purpose

This PR fixes deletion for collections with an associated challenge (gift exchange or prompt meme) by destroying the challenge first before the collection itself.

Credit

Aya Sayadi

[redsummernight]

I think we can fix this just with callbacks. In collection, you can keep the association as-is:

otwarchive/app/models/collection.rb

Line 34 in e468bce

belongs_to :challenge, dependent: :destroy, polymorphic: true

In both challenge models, you can update the association to use :nullify.

has_one :collection, as: :challenge, dependent: :nullify

(which means you can fully remove all references of clear_challenge_references).

Rails 5+ should allow this bi-directional association to work (rails/rails#21350).

[ayasayadi1]

Hi, thank you for reviewing!

dependent: :nullify clears the FK. But since we now bypass ActiveRecord callbacks, clean_up_challenge never fires. As a result, signups, assignments, prompts etc survive the deletion. Some GE and PM feature tests caught this.

Previously, the flow was:

1. before_destroy :clear_challenge_references -> challenge references cleared -> collection.save!
2. after_save :clean_up_challenge -> associated records deleted
3. challenge deleted

Now it's:

1. challenge deleted
2. :nullify -> challenge references cleared
3. associated records remain

I worked around this by moving clean_up_challenge into ChallengeCore and calling it via before_destroy, which restores the original behaviour

That said, I think this workflow is still risky: if the challenge deletion fails after before_destroy runs, we've already lost the associated records. Please lmk what you think :)

[redsummernight]

You should use destroy! in specs, that way if there are errors you can easily see it in test logs.

[redsummernight]

Can you also update gift_exchange_spec.rb and prompt_meme_spec.rb to test that a collection's challenge_id and challenge_type fields are nulled when the associated challenge is deleted?