use a subquery to speed up the last messages query (#3)

* use a subquery to speed up the last messages query
* fix style recommendations from rubocoop

plugins/messages/app/models/message.rb

@@ -18,6 +18,13 @@ class Message < ApplicationRecord
       .where('private = ? OR sender_id = ? OR message_recipients.user_id = ?', false, user_id, user_id)
       .distinct
   }
+  scope :last_readable_for, ->(user, nr) {
+    user_id = user.try(&:id)
+    received = MessageRecipient.select(:message_id, :user_id).where(user_id: user_id).limit(nr)
+
+    joins("LEFT OUTER JOIN (#{received.to_sql}) sub ON messages.id = sub.message_id")
+      .where('private = ? OR sender_id = ? OR sub.user_id = ?', false, user_id, user_id)
+  }
 
   validates_presence_of :message_recipients, :subject, :body
   validates_length_of :subject, :in => 1..255

plugins/messages/app/overrides/home/index/latest_public_messages_index.html.haml.deface

@@ -3,5 +3,5 @@
   - unless Message.readable_for(current_user).empty?
     %section#messages
       %h2= t '.messages.title'
-      = render 'messages/messages', messages: Message.readable_for(current_user).includes(:sender).order('created_at DESC').limit(5), pagination: false
+      = render 'messages/messages', messages: Message.last_readable_for(current_user,5).includes(:sender).order('created_at DESC').limit(5), pagination: false
       %p= raw t '.messages.view_all.text', messages: link_to(t('.messages.view_all.messages'), messages_path), threads: link_to(t('.messages.view_all.threads'), message_threads_path)