Add a few more indexes, make some explicit queries to make sure we're using those indexes.

models/comment.rb

@@ -15,6 +15,8 @@ class Comment < Content
   field :anonymous, type: Boolean, default: false
   field :at_position_list, type: Array, default: []
 
+  index({author_id: 1, course_id: 1})
+
   belongs_to :comment_thread, index: true
   belongs_to :author, class_name: "User", index: true
 

models/comment_thread.rb

@@ -21,6 +21,8 @@ class CommentThread < Content
   field :at_position_list, type: Array, default: []
   field :last_activity_at, type: Time
 
+  index({author_id: 1, course_id: 1})
+
   include Tire::Model::Search
   include Tire::Model::Callbacks
 

models/subscription.rb

@@ -7,6 +7,8 @@ class Subscription
   field :source_type, type: String
   
   index({subscriber_id: 1, source_id: 1, source_type: 1})
+  index({subscriber_id: 1, source_type: 1})
+  index({subscriber_id: 1})
 
   def to_hash
     as_document.slice(*%w[subscriber_id source_id source_type])

models/user.rb

@@ -31,7 +31,7 @@ class User
   end
 
   def subscribed_thread_ids
-    subscriptions_as_subscriber.where(source_type: "CommentThread").only(:source_id).map(&:source_id)
+    Subscription.where(subscriber_id: id.to_s, source_type: "CommentThread").only(:source_id).map(&:source_id)
   end
 
   def subscribed_commentable_ids
@@ -47,7 +47,7 @@ class User
   end
 
   def subscribed_commentables
-    Commentable.where(:id.in => subscribed_commentable_ids).only(:id).map(&:id)
+    Commentable.find(*subscribed_commentable_ids).only(:id).map(&:id)
   end
 
   def subscribed_users
@@ -64,11 +64,12 @@ class User
                         "id" => id,
                         "upvoted_ids" => upvoted_ids,
                         "downvoted_ids" => downvoted_ids,
-                        "default_sort_key" => default_sort_key)
+                        "default_sort_key" => default_sort_key
+                       )
     end
     if params[:course_id]
-      hash = hash.merge("threads_count" => comment_threads.where(course_id: params[:course_id]).count,
-                        "comments_count" => comments.where(course_id: params[:course_id]).count,
+      hash = hash.merge("threads_count" => CommentThread.where(user_id: id, course_id: params[:course_id]).count,
+                        "comments_count" => Comment.where(user_id: id, course_id: params[:course_id]).count
                        )
     end
     hash