Eager-load comment user data (#14902)

* Eager-load comment user data

Profiles and user settings are loaded for on each comment author. On
posts with many comments, this invokes a *lot* of DB queries. This
commit replaces 2 queries per comment with 2 queries for all comments.

* Fix where I misunderstood limit's purpose

This implementation is too clever. It looks like it should just be
getting all but the last, but it uses the fact that the default is 0 to
default to getting all of them. This isn't readily understandable to
someone reading the code.

I don't have a suggestion to fix it yet so I'm just going to leave it
as-is for now.

app/models/comment.rb

@@ -91,7 +91,11 @@ class Comment < ApplicationRecord
   alias touch_by_reaction save
 
   def self.tree_for(commentable, limit = 0)
-    commentable.comments.includes(:user).arrange(order: "score DESC").to_a[0..limit - 1].to_h
+    commentable.comments
+      .includes(user: %i[setting profile])
+      .arrange(order: "score DESC")
+      .to_a[0..limit - 1]
+      .to_h
   end
 
   def search_id

app/views/comments/index.html.erb

@@ -124,7 +124,7 @@
     <div class="comments" id="comment-trees-container">
       <% if @root_comment.present? %>
         <% cache ["comment_root-view-root_#{user_signed_in?}", @root_comment] do %>
-          <%= tree_for(@root_comment, @root_comment.subtree.includes(:user).arrange[@root_comment], @commentable) %>
+          <%= tree_for(@root_comment, @root_comment.subtree.includes(user: %i[setting profile]).arrange[@root_comment], @commentable) %>
         <% end %>
       <% else %>
         <% Comment.tree_for(@commentable).each do |comment, sub_comments| %>