Deleting a PolicyGroup does not delete its associated attachments

[floehopper]

This is inconsistent with what happens when you discard a draft edition. In that case, all the edition's attachments are marked as deleted and they are no longer visible, even in the draft stack:

• The Admin::EditionsController has a #destroy action
• This action is accessible via the "Discard draft" button
• This action uses an instance of EditionDeleter which has a #fire_transition! method which, in turn, calls Attachable#delete_all_attachments which marks the attachments as deleted
• This deleted flag is used to decide whether an AttachmentData should be visible

Conversely:

• Admin::PolicyGroupsController has a #destroy action
• This action is available to GDS Editors on the policy group index page
• The PolicyGroup model has no custom attachment-related behaviour other than that added by Attachable
• Attachable doesn't have any callbacks which would mark attachments as deleted

[floehopper]

Having just thought about this a bit more, I'm not sure the attachments should be marked as deleted when a policy group is deleted.

A policy group is immediately public when it is created. Similarly attachments on a policy group are immediately public when they are added to the policy group.

If we think of policy group creation as the equivalent of the publishing of an edition, and policy group destruction as the equivalent of the un-publishing of an edition, it could be argued that the attachments should stay visible when the policy group is destroyed. Or at least the attachment URLs should still do something sensible, e.g. redirect to some kind of "un-publishing page".

It looks as if policy groups are published via the Publishing API, so perhaps there is a mechanism to display some kind of "un-publishing page" when a policy group is destroyed.

Anyway, I'm not planning to look into this any further at this stage.