Post#pools: avoid query when post has no pools.

app/models/post.rb

@@ -1026,8 +1026,9 @@ class Post < ActiveRecord::Base
   module PoolMethods
     def pools
       @pools ||= begin
+        return Pool.none if pool_string.blank?
         pool_ids = pool_string.scan(/\d+/)
-        Pool.where(["is_deleted = false and id in (?)", pool_ids])
+        Pool.undeleted.where(id: pool_ids).series_first
       end
     end