PERF: avoid expensive order by random for suggested topics

spec/services/random_topic_selector_spec.rb

@@ -0,0 +1,27 @@
+require 'spec_helper'
+
+describe RandomTopicSelector do
+
+  it 'can correctly use cache' do
+    key = RandomTopicSelector.cache_key
+
+    $redis.del key
+
+    4.times do |t|
+      $redis.rpush key, t
+    end
+
+    RandomTopicSelector.next(2).should == [0,1]
+    RandomTopicSelector.next(2).should == [2,3]
+  end
+
+  it 'can correctly backfill' do
+    category = Fabricate(:category)
+    t1 = Fabricate(:topic, category_id: category.id)
+    _t2 = Fabricate(:topic, category_id: category.id, visible: false)
+    _t3 = Fabricate(:topic, category_id: category.id, deleted_at: 1.minute.ago)
+    t4 = Fabricate(:topic, category_id: category.id)
+
+    RandomTopicSelector.next(5, category).sort.should == [t1.id,t4.id].sort
+  end
+end