category_list_avoid_loading_subcategories_patch.diff

diff --git a/app/models/category_list.rb b/app/models/category_list.rb
index ef8e78329e..d106fcead6 100644
--- a/app/models/category_list.rb
+++ b/app/models/category_list.rb
@@ -41,19 +41,6 @@ class CategoryList
     "categories_list"
   end
 
-  def self.order_categories(categories)
-    if SiteSetting.fixed_category_positions
-      categories.order(:position, :id)
-    else
-      allowed_category_ids = categories.pluck(:id) << nil # `nil` is necessary to include categories without any associated topics
-      categories.left_outer_joins(:featured_topics)
-        .where(topics: { category_id: allowed_category_ids })
-        .group('categories.id')
-        .order("max(topics.bumped_at) DESC NULLS LAST")
-        .order('categories.id ASC')
-    end
-  end
-
   private
 
   def find_relevant_topics
@@ -101,58 +88,71 @@ class CategoryList
       :uploaded_logo,
       :uploaded_logo_dark,
       :topic_only_relative_url,
-      subcategories: [:topic_only_relative_url]
     ).secured(@guardian)
 
+    include_subcategories = @options[:include_subcategories] == true
+
+    if include_subcategories
+      @categories = @categories.includes(subcategories: [:topic_only_relative_url])
+    else
+      @categories = @categories.where("categories.parent_category_id IS NULL")
+    end
+
     @categories = @categories.where("categories.parent_category_id = ?", @options[:parent_category_id].to_i) if @options[:parent_category_id].present?
 
-    @categories = self.class.order_categories(@categories)
+    @categories = order_categories(@categories)
 
     @categories = @categories.to_a
 
-    include_subcategories = @options[:include_subcategories] == true
-
     notification_levels = CategoryUser.notification_levels_for(@guardian.user)
     default_notification_level = CategoryUser.default_notification_level
 
     if @options[:parent_category_id].blank?
-      subcategory_ids = {}
       subcategory_list = {}
       to_delete = Set.new
+
       @categories.each do |c|
-        if c.parent_category_id.present?
-          subcategory_ids[c.parent_category_id] ||= []
-          subcategory_ids[c.parent_category_id] << c.id
-          if include_subcategories
-            subcategory_list[c.parent_category_id] ||= []
-            subcategory_list[c.parent_category_id] << c
-          end
-          to_delete << c
+        if c.parent_category_id.present? && include_subcategories
+          subcategory_list[c.parent_category_id] ||= []
+          subcategory_list[c.parent_category_id] << c
+          to_delete << c.id
         end
       end
+
       @categories.each do |c|
-        c.subcategory_ids = subcategory_ids[c.id] || []
+        c.subcategory_ids =
+          if c.parent_category_id
+            Category.subcategory_ids(c.id) - [c.id] # Category.subcategory_ids returns the category id as well
+          else
+            []
+          end
+
         if include_subcategories
           c.subcategory_list = subcategory_list[c.id] || []
         end
       end
-      @categories.delete_if { |c| to_delete.include?(c) }
+
+      @categories.delete_if { |c| to_delete.include?(c.id) }
     end
 
     allowed_topic_create = Set.new(Category.topic_create_allowed(@guardian).pluck(:id))
+
     categories_with_descendants.each do |category|
       category.notification_level = notification_levels[category.id] || default_notification_level
       category.permission = CategoryGroup.permission_types[:full] if allowed_topic_create.include?(category.id)
-      category.has_children = category.subcategories.present?
+      category.has_children = category.subcategory_ids.present?
     end
 
     if @topics_by_category_id
       categories_with_descendants.each do |c|
         topics_in_cat = @topics_by_category_id[c.id]
+
         if topics_in_cat.present?
           c.displayable_topics = []
+
           topics_in_cat.each do |topic_id|
             topic = @topics_by_id[topic_id]
+
             if topic.present? && @guardian.can_see?(topic)
               # topic.category is very slow under rails 4.2
               topic.association(:category).target = c
@@ -164,6 +164,17 @@ class CategoryList
     end
   end
 
+  def order_categories(categories)
+    if SiteSetting.fixed_category_positions
+      categories.order(:position, :id)
+    else
+      categories.left_outer_joins(:featured_topics)
+        .group('categories.id')
+        .order("max(topics.bumped_at) DESC NULLS LAST")
+        .order('categories.id ASC')
+    end
+  end
+
   def prune_empty
     return if SiteSetting.allow_uncategorized_topics
     @categories.delete_if { |c| c.uncategorized? }
@@ -210,8 +221,14 @@ class CategoryList
     return @categories_with_children if @categories_with_children && (categories == @categories)
     return nil if categories.nil?
 
-    result = categories.flat_map do |c|
-      [c, *categories_with_descendants(c.subcategory_list)]
+    result = categories.flat_map do |category|
+      categories = [category]
+
+      if category.subcategory_list
+        categories.push(*categories_with_descendants(category.subcategory_list))
+      end
+
+      categories
     end
 
     @categories_with_children = result if categories == @categories