PERF: speed up query

app/models/post_action.rb

@@ -82,18 +82,26 @@ class PostAction < ActiveRecord::Base
 
   def self.lookup_for(user, topics, post_action_type_id)
     return if topics.blank?
-
+    # in critical path 2x faster than AR
+    #
+    topic_ids = topics.map(&:id)
     map = {}
-    PostAction.where(user_id: user.id, post_action_type_id: post_action_type_id, deleted_at: nil)
-              .references(:post)
-              .includes(:post)
-              .where('posts.topic_id in (?)', topics.map(&:id))
-              .order('posts.topic_id, posts.post_number')
-              .pluck('posts.topic_id, posts.post_number')
-              .each do |topic_id, post_number|
-                (map[topic_id] ||= []) << post_number
+        builder = SqlBuilder.new <<SQL
+        SELECT p.topic_id, p.post_number
+        FROM post_actions pa
+        JOIN posts p ON pa.post_id = p.id
+        WHERE p.deleted_at IS NULL AND pa.deleted_at IS NULL AND
+           pa.post_action_type_id = :post_action_type_id AND
+           pa.user_id = :user_id AND
+           p.topic_id IN (:topic_ids)
+        ORDER BY p.topic_id, p.post_number
+SQL
+
+    builder.map_exec(OpenStruct, user_id: user.id, post_action_type_id: post_action_type_id, topic_ids: topic_ids).each do |row|
+      (map[row.topic_id] ||= []) << row.post_number
     end
 
+
     map
   end
 

spec/models/post_action_spec.rb

@@ -490,6 +490,18 @@ describe PostAction do
 
   end
 
+  describe ".lookup_for" do
+    it "returns the correct map" do
+      user = Fabricate(:user)
+      post = Fabricate(:post)
+      post_action = PostAction.create(user_id: user.id, post_id: post.id, post_action_type_id: 1)
+
+      map = PostAction.lookup_for(user, [post.topic], post_action.post_action_type_id)
+
+      expect(map).to eq({post.topic_id => [post.post_number]})
+    end
+  end
+
   describe ".add_moderator_post_if_needed" do
 
     it "should not add a moderator post when it's disabled" do