fixes #2192

app/models/forum_topic.rb

@@ -124,7 +124,7 @@ class ForumTopic < ActiveRecord::Base
     end
     hash[id.to_s] = updated_at.to_i.to_s
     result = hash.to_a.flatten.join(" ")
-    while result.size > 3000
+    while result.size > 500
       ids = result.scan(/\S+/)
       result = ids[(ids.size / 2)..-1].join(" ")
     end

test/unit/forum_topic_test.rb

@@ -110,7 +110,7 @@ class ForumTopicTest < ActiveSupport::TestCase
       should "prune the string if it gets too long" do
         array = (1..1_000).to_a.map(&:to_s).in_groups_of(2)
         result = @topic.mark_as_read(array)
-        assert_operator result.size, :<, 3000
+        assert_operator result.size, :<, 500
       end
     end