FIX: A memoization bug in UserLookup and refactor (#13692)

`@group_lookup` memoization bug was introduced in #13587.

lib/user_lookup.rb

@@ -1,6 +1,13 @@
 # frozen_string_literal: true
 
 class UserLookup
+  def self.lookup_columns
+    @user_lookup_columns ||= %i{id username name uploaded_avatar_id primary_group_id flair_group_id admin moderator trust_level}
+  end
+
+  def self.group_lookup_columns
+    @group_lookup_columns ||= %i{id name flair_icon flair_upload_id flair_bg_color flair_color}
+  end
 
   def initialize(user_ids = [])
     @user_ids = user_ids.tap(&:compact!).tap(&:uniq!).tap(&:flatten!)
@@ -12,66 +19,42 @@ class UserLookup
   end
 
   def primary_groups
-    @primary_groups ||= begin
-      hash = {}
-      users.values.each do |u|
-        if u.primary_group_id
-          hash[u.id] = groups[u.primary_group_id]
-        end
+    @primary_groups ||= users.values.each_with_object({}) do |user, hash|
+      if user.primary_group_id
+        hash[user.id] = groups[user.primary_group_id]
       end
-      hash
     end
   end
 
   def flair_groups
-    @flair_groups ||= begin
-      hash = {}
-      users.values.each do |u|
-        if u.flair_group_id
-          hash[u.id] = groups[u.flair_group_id]
-        end
+    @flair_groups ||= users.values.each_with_object({}) do |user, hash|
+      if user.flair_group_id
+        hash[user.id] = groups[user.flair_group_id]
       end
-      hash
     end
   end
 
   private
 
-  def self.lookup_columns
-    @user_lookup_columns ||= %i{id username name uploaded_avatar_id primary_group_id flair_group_id admin moderator trust_level}
-  end
-
-  def self.group_lookup_columns
-    @group_lookup_columns ||= %i{id name flair_icon flair_upload_id flair_bg_color flair_color}
-  end
-
   def users
-    @users ||= user_lookup_hash
-  end
-
-  def user_lookup_hash
-    hash = {}
-    User.where(id: @user_ids)
+    @users ||= User
+      .where(id: @user_ids)
       .select(self.class.lookup_columns)
-      .each { |user| hash[user.id] = user }
-    hash
+      .index_by(&:id)
   end
 
   def groups
-    @group_lookup = begin
+    @group_lookup ||= begin
       group_ids = users.values.map { |u| [u.primary_group_id, u.flair_group_id] }
       group_ids.flatten!
       group_ids.uniq!
       group_ids.compact!
 
-      hash = {}
-
-      Group.includes(:flair_upload)
+      Group
+        .includes(:flair_upload)
         .where(id: group_ids)
         .select(self.class.group_lookup_columns)
-        .each { |g| hash[g.id] = g }
-
-      hash
+        .index_by(&:id)
     end
   end
 end