Reduced merges, do them in place, and reduce allocations.

Primarily, we wanted to reduce the one-thing-per-merge calls, but also,
we want to reduce allocations overall, both from merge calls which will
duplicate the source hash and from string constants sprinkled
throughout.

Seemingly, defining a constant-style word list is not the ticket.  For
whatever reason, Ruby can't seem to figure out the list doesn't change,
and presumably is splating out all of the words and allocating them as
if they were declared right there.  Wahhhh.

Now we're using a straight up one-for-one constant definition of each
string, in a big constants file, where we freeze the strings on the
spot.  We then reference each constant directly at the callsite, which
was the original style and, ultimately, is the best for readability.

models/comment.rb

 require_relative 'content'
+require_relative 'constants'
 
 class Comment < Content
-
   include Mongoid::Tree
   include Mongoid::Timestamps
   include Mongoid::MagicCounterCache
@@ -25,7 +25,6 @@ class Comment < Content
   index({author_id: 1, course_id: 1})
   index({_type: 1, comment_thread_id: 1, author_id: 1, updated_at: 1})
 
-
   index_name Content::ES_INDEX_NAME
 
   mapping do
@@ -39,7 +38,6 @@ class Comment < Content
     indexes :updated_at, type: :date, included_in_all: false
   end
 
-
   belongs_to :comment_thread, index: true
   belongs_to :author, class_name: 'User', index: true
 
@@ -91,19 +89,20 @@ class Comment < Content
       subtree_hash = subtree(sort: sort_by_parent_and_time)
       self.class.hash_tree(subtree_hash).first
     else
-      as_document.slice(*%w[body course_id endorsed endorsement anonymous anonymous_to_peers created_at updated_at at_position_list])
-          .merge("id" => _id)
-          .merge("user_id" => author_id)
-          .merge("username" => author_username)
-          .merge("depth" => depth)
-          .merge("closed" => comment_thread.nil? ? false : comment_thread.closed)
-          .merge("thread_id" => comment_thread_id)
-          .merge("parent_id" => parent_ids[-1])
-          .merge("commentable_id" => comment_thread.nil? ? nil : comment_thread.commentable_id)
-          .merge("votes" => votes.slice(*%w[count up_count down_count point]))
-          .merge("abuse_flaggers" => abuse_flaggers)
-          .merge("type" => "comment")
-          .merge("child_count" => get_cached_child_count)
+      as_document
+        .slice(BODY, COURSE_ID, ENDORSED, ENDORSEMENT, ANONYMOUS, ANONYMOUS_TO_PEERS, CREATED_AT, UPDATED_AT, AT_POSITION_LIST)
+        .merge!("id" => _id,
+                "user_id" => author_id,
+                "username" => author_username,
+                "depth" => depth,
+                "closed" => comment_thread.nil? ? false : comment_thread.closed,
+                "thread_id" => comment_thread_id,
+                "parent_id" => parent_ids[-1],
+                "commentable_id" => comment_thread.nil? ? nil : comment_thread.commentable_id,
+                "votes" => votes.slice(COUNT, UP_COUNT, DOWN_COUNT, POINT),
+                "abuse_flaggers" => abuse_flaggers,
+                "type" => COMMENT,
+                "child_count" => get_cached_child_count)
     end
   end
 

models/comment_thread.rb

-# -*- coding: utf-8 -*-
 require 'new_relic/agent/method_tracer'
 require_relative 'content'
+require_relative 'constants'
 
 class CommentThread < Content
-
   include Mongoid::Timestamps
   include Mongoid::Attributes::Dynamic
   include ActiveModel::MassAssignmentSecurity
@@ -32,7 +31,6 @@ class CommentThread < Content
 
   index({author_id: 1, course_id: 1})
 
-
   index_name Content::ES_INDEX_NAME
 
   mapping do
@@ -122,17 +120,18 @@ class CommentThread < Content
   end
 
   def to_hash(params={})
-    as_document.slice(*%w[thread_type title body course_id anonymous anonymous_to_peers commentable_id created_at updated_at at_position_list closed context last_activity_at])
-        .merge('id' => _id,
-               'user_id' => author_id,
-               'username' => author_username,
-               'votes' => votes.slice(*%w[count up_count down_count point]),
-               'abuse_flaggers' => abuse_flaggers,
-               'tags' => [],
-               'type' => 'thread',
-               'group_id' => group_id,
-               'pinned' => pinned?,
-               'comments_count' => comment_count)
+    as_document
+      .slice(THREAD_TYPE, TITLE, BODY, COURSE_ID, ANONYMOUS, ANONYMOUS_TO_PEERS, COMMENTABLE_ID, CREATED_AT, UPDATED_AT, AT_POSITION_LIST, CLOSED, CONTEXT, LAST_ACTIVITY_AT)
+      .merge!("id" => _id,
+              "user_id" => author_id,
+              "username" => author_username,
+              "votes" => votes.slice(COUNT, UP_COUNT, DOWN_COUNT, POINT),
+              "abuse_flaggers" => abuse_flaggers,
+              "tags" => [],
+              "type" => THREAD,
+              "group_id" => group_id,
+              "pinned" => pinned?,
+              "comments_count" => comment_count)
 
   end
 

models/constants.rb

+BODY = "body".freeze
+COURSE_ID = "course_id".freeze
+ENDORSED = "endorsed".freeze
+ENDORSEMENT = "endorsement".freeze
+ANONYMOUS = "anonymous".freeze
+ANONYMOUS_TO_PEERS = "anonymous_to_peers".freeze
+CREATED_AT = "created_at".freeze
+UPDATED_AT = "updated_at".freeze
+AT_POSITION_LIST = "at_position_list".freeze
+THREAD_TYPE = "thread_type".freeze
+TITLE = "title".freeze
+COMMENTABLE_ID = "commentable_id".freeze
+CLOSED = "closed".freeze
+CONTEXT = "context".freeze
+LAST_ACTIVITY_AT = "last_activity_at".freeze
+NOTIFICATION_TYPE = "notification_type".freeze
+INFO = "info".freeze
+ACTOR_ID = "actor_id".freeze
+TARGET_ID = "target_id".freeze
+SUBSCRIBER_ID = "subscriber_id".freeze
+SOURCE_ID = "source_id".freeze
+SOURCE_TYPE = "source_type".freeze
+COUNT = "count".freeze
+UP_COUNT = "up_count".freeze
+DOWN_COUNT = "down_count".freeze
+POINT = "point".freeze
+USERNAME = "username".freeze
+EXTERNAL_ID = "external_id".freeze
+COMMENT = "comment".freeze
+THREAD = "thread".freeze

models/notification.rb

+require_relative 'constants'
+
 class Notification
   include Mongoid::Document
   include Mongoid::Timestamps
@@ -14,6 +16,8 @@ class Notification
   has_and_belongs_to_many :receivers, class_name: "User", inverse_of: :notifications, autosave: true
 
   def to_hash(params={})
-    as_document.slice(*%w[notification_type info actor_id target_id]).merge("id" => _id)
+    as_document
+      .slice(NOTIFICATION_TYPE, INFO, ACTOR_ID, TARGET_ID)
+      .merge!("id" => _id)
   end
 end

models/subscription.rb

+require_relative 'constants'
+
 class Subscription
   include Mongoid::Document
   include Mongoid::Timestamps
@@ -12,7 +14,9 @@ class Subscription
   index({source_id: 1, source_type: 1}, {background: true})
 
   def to_hash
-    as_document.slice(*%w[subscriber_id source_id source_type]).merge("id" => _id)
+    as_document
+      .slice(SUBSCRIBER_ID, SOURCE_ID, SOURCE_TYPE)
+      .merge!("id" => _id)
   end
 
   def subscriber
@@ -22,5 +26,4 @@ class Subscription
   def source
     source_type.constantize.find(source_id)
   end
-
 end

models/user.rb

 require 'new_relic/agent/method_tracer'
+require_relative 'constants'
 
 class User
   include Mongoid::Document
@@ -35,18 +36,20 @@ class User
   end
 
   def to_hash(params={})
-    hash = as_document.slice(*%w[username external_id])
+    hash = as_document
+      .slice(USERNAME, EXTERNAL_ID)
+
     if params[:complete]
-      hash = hash.merge("subscribed_thread_ids" => subscribed_thread_ids,
+      hash = hash.merge!("subscribed_thread_ids" => subscribed_thread_ids,
                         "subscribed_commentable_ids" => [], # not used by comment client.  To be removed once removed from comment client.
                         "subscribed_user_ids" => [], # ditto.
                         "follower_ids" => [], # ditto.
                         "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]
       self.class.trace_execution_scoped(['Custom/User.to_hash/count_comments_and_threads']) do
         if not params[:group_ids].empty?
@@ -101,7 +104,7 @@ class User
             anonymous_to_peers: false
           ).reject{ |comment| comment.standalone_context? }.count
         end
-        hash = hash.merge("threads_count" => threads_count, "comments_count" => comments_count)
+        hash = hash.merge!("threads_count" => threads_count, "comments_count" => comments_count)
       end
     end
     hash

presenters/thread.rb

@@ -97,7 +97,7 @@ class ThreadPresenter
     top_level = []
     ancestry = []
     content.each do |item|
-      item_hash = item.to_hash.merge("children" => [])
+      item_hash = item.to_hash.merge!("children" => [])
       if item.parent_id.nil?
         top_level << item_hash
         ancestry = [item_hash]