PERF: Don't join on shared drafts unless you have to

app/controllers/list_controller.rb

@@ -71,7 +71,10 @@ class ListController < ApplicationController
 
       list = TopicQuery.new(user, list_opts).public_send("list_#{filter}")
 
-      if @category.present? && guardian.can_create_shared_draft?
+      if @category.present? &&
+        guardian.can_create_shared_draft? &&
+        @category.id != SiteSetting.shared_drafts_category.to_i
+
         shared_drafts = TopicQuery.new(
           user,
           category: SiteSetting.shared_drafts_category,

app/models/topic_list.rb

@@ -26,18 +26,21 @@ class TopicList
     end
   end
 
-  attr_accessor :more_topics_url,
-                :prev_topics_url,
-                :draft,
-                :draft_key,
-                :draft_sequence,
-                :filter,
-                :for_period,
-                :per_page,
-                :top_tags,
-                :current_user,
-                :tags,
-                :shared_drafts
+  attr_accessor(
+    :more_topics_url,
+    :prev_topics_url,
+    :draft,
+    :draft_key,
+    :draft_sequence,
+    :filter,
+    :for_period,
+    :per_page,
+    :top_tags,
+    :current_user,
+    :tags,
+    :shared_drafts,
+    :category
+  )
 
   def initialize(filter, current_user, topics, opts = nil)
     @filter = filter

app/serializers/topic_list_item_serializer.rb

@@ -1,6 +1,8 @@
 class TopicListItemSerializer < ListableTopicSerializer
   include TopicTagsMixin
 
+  attr_accessor :include_destination_category
+
   attributes :views,
              :like_count,
              :has_summary,
@@ -30,8 +32,9 @@ class TopicListItemSerializer < ListableTopicSerializer
   end
 
   def category_id
+
     # If it's a shared draft, show the destination topic instead
-    if object.category_id == SiteSetting.shared_drafts_category.to_i && object.shared_draft
+    if include_destination_category && object.shared_draft
       return object.shared_draft.category_id
     end
 

app/serializers/topic_list_serializer.rb

@@ -9,12 +9,26 @@ class TopicListSerializer < ApplicationSerializer
              :per_page,
              :top_tags,
              :tags,
-             :shared_drafts
+             :shared_drafts,
+             :topics
 
-  has_many :topics, serializer: TopicListItemSerializer, embed: :objects
   has_many :shared_drafts, serializer: TopicListItemSerializer, embed: :objects
   has_many :tags, serializer: TagSerializer, embed: :objects
 
+  def topics
+    object.topics.map do |t|
+      serializer = TopicListItemSerializer.new(t, scope: scope, root: false)
+
+      if scope.can_create_shared_draft? &&
+        object.category&.id == SiteSetting.shared_drafts_category.to_i
+
+        serializer.include_destination_category = true
+      end
+
+      serializer
+    end
+  end
+
   def can_create_topic
     scope.can_create?(Topic)
   end

lib/topic_query.rb

@@ -482,17 +482,19 @@ class TopicQuery
     end
 
     def apply_shared_drafts(result, category_id, options)
-      viewing_shared = category_id && category_id == SiteSetting.shared_drafts_category.to_i
+      drafts_category_id = SiteSetting.shared_drafts_category.to_i
+      viewing_shared = category_id && category_id == drafts_category_id
 
       if guardian.can_create_shared_draft?
-        result = result.includes(:shared_draft).references(:shared_draft)
-
         if options[:destination_category_id]
           destination_category_id = get_category_id(options[:destination_category_id])
-          return result.where("shared_drafts.category_id" => destination_category_id)
+          topic_ids = SharedDraft.where(category_id: destination_category_id).pluck(:topic_id)
+          return result.where(id: topic_ids)
+        elsif viewing_shared
+          result = result.includes(:shared_draft).references(:shared_draft)
+        else
+          return result.where('topics.category_id != ?', drafts_category_id)
         end
-
-        return result.where("shared_drafts.id IS NULL") unless viewing_shared
       end
 
       result