When chat is enabled, there's a scheduled job that runs every 5 minutes to check whether we need to send a "chat summary" email to users with unread chat messages or mentions.
On Discourse with a large number of users, the query used wasn't optimal and sometimes taking minutes. Which isn't good when the query is called every 5 minutes 😬
This PR reworks the query in `Chat::Mailer.send_unread_mentions_summary`.
Instead of starting from the `users` table, it starts from the `user_chat_channel_memberships` table which is the main piece tying everything together.
The new query is mostly similar to the previous one, with some bug fixes (like ensuring the user has `allow_private_messages` enabled for direct messages) and is also slightly simpler since it doesn't keep track of the `memberships_with_unread_messages` anymore. That part has been moved to the `user_notifications.chat_summary` email method.
The `UserEmailExtension` has been deleted since that was using to N+1 update the `user_chat_channel_memberships.last_unread_mention_when_emailed_it`(quite a mouthful 😛) but that's now done directly in the `user_notifications.chat_summary` email method.
The "plat de résistance" of that PR - the `user_notifications.chat_summary` method has been re-worked for improved performances 🚀
Instead of doing everything in one query, it does 4 tiny ones.
- One to retrieve the list of unread mentions (@something) in "category" channels
- One to retrieve the list of unread messages in "direct message" channels (aka. 1-1 and group discussions)
- One to load all the chat messages for each "category" channels from the last unread mention
- One to load all the chat messages for each "direct message" channels from the last unread message
All the specs for both `Chat::Mailer` and `UserNotification.chat_summary` have been rewriten for easier comprehension and faster execution (mostly by not using chat services which makes the specs go 10x slower...)
Internal ref - t/129848
Chat will now check for the state of `SiteSetting.private_email` when sending the summary, when enabled, the mail will not display user information, channel information other than the ID and no message information, only the count of messages.
Régis Hanol
Kris
Joffrey JAFFEUX
Roman Rizzi