Poor performance in EmailAdmin view.

[barqshasbite]

There is a performance issue when viewing an Email in the EmailAdmin when there are a lot Emails and Attachments in the database. This is caused by customizing the get_queryset method of AttachmentInline to try to filter out inline attachments by their header values.

In the post_office.admin.AttachmentInline.get_queryset method, Django has not yet applied the Email FK filter to the queryset. That happens later when the inline formset is being created.

That means that the query being run in the get_queryset method actually fetches and iterates over every single Email Attachment object in the database, regardless of the Email it is for. In the loop, it also references the attachment property, but without a select_related on the query, it will have to fetch each attachment separately for every iteration.

If you have a lot of Emails and Attachments in the database, this causes performance issues.

[patroqueeet]

I'd like to add for many users in db, the AttachmentAdmin is very slow to load.