The #pluck_first freedom patch, first introduced by @danielwaterworth has served us well, and is used widely throughout both core and plugins. It seems to have been a common enough use case that Rails 6 introduced it's own method #pick with the exact same implementation. This allows us to retire the freedom patch and switch over to the built-in ActiveRecord method.
There is no replacement for #pluck_first!, but a quick search shows we are using this in a very limited capacity, and in some cases incorrectly (by assuming a nil return rather than an exception), which can quite easily be replaced with #pick plus some extra handling.
In order to include the new/unread count in the browse more message
under suggested topics, a couple of technical changes have to be made.
1. `PrivateMessageTopicTrackingState` is now auto-injected which is
similar to how it is done for `TopicTrackingState`. This is done so
we don't have to attempt to pass the `PrivateMessageTopicTrackingState`
object multiple levels down into the suggested-topics component. While
the object is auto-injected, we only fetch the initial state and start
tracking when the relevant private messages routes has been hit and only
when a private message's suggested topics is loaded. This is
done as we do not want to add the extra overhead of fetching the inital
state to all page loads but instead wait till the private messages
routes are hit.
2. Previously, we would stop tracking once the `user-private-messages`
route has been deactivated. However, that is not ideal since
navigating out of the route and back means we send an API call to the
server each time. Since `PrivateMessageTopicTrackingState` is kept in
sync cheaply via messageBus, we can just continue to track the state
even if the user has navigated away from the relevant stages.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
