refactor channel rankings

db/migrate/20140801190147_set_rankings_for_channels.rb

@@ -0,0 +1,8 @@
+class SetRankingsForChannels < ActiveRecord::Migration
+  def change
+    Channel.find_each do |channel|
+      channel.set_ranking
+    end
+  end
+end
+

db/migrate/20140801191621_modify_ranking_index_on_channels.rb

@@ -0,0 +1,7 @@
+class ModifyRankingIndexOnChannels < ActiveRecord::Migration
+  def change
+    remove_index :channels, :ranking
+    add_index :channels, [:ranking, :updated_at]
+  end
+end
+