diff --git a/app/services/random_topic_selector.rb b/app/services/random_topic_selector.rb
index 7c2026b5e..d412a2389 100644
--- a/app/services/random_topic_selector.rb
+++ b/app/services/random_topic_selector.rb
@@ -69,10 +69,7 @@ class RandomTopicSelector
   end
 
   def self.clear_cache!
-    Category.select(:id).each do |c|
-      $redis.del cache_key(c)
-    end
-    $redis.del cache_key
+    $redis.keys('random_topic_cache*').each{|k| $redis.del k}
   end
 
   def self.cache_key(category=nil)