track when tag subscriptions are used (for pruning)

app/models/tag_subscription.rb

@@ -72,6 +72,10 @@ class TagSubscription < ActiveRecord::Base
     if name
       relation = relation.where(["name ILIKE ? ESCAPE E'\\\\'", name.to_escaped_for_sql_like])
     end
+    
+    relation.each do |tag_sub|
+      tag_sub.update_column(:last_accessed_at, Time.now)
+    end
 
     relation.map {|x| x.post_ids.split(/,/)}.flatten.uniq.map(&:to_i).sort.reverse.slice(0, limit)
   end

db/migrate/20100307073438_create_tag_subscriptions.rb

@@ -6,6 +6,7 @@ class CreateTagSubscriptions < ActiveRecord::Migration
       t.column :tag_query, :string, :null => false
       t.column :post_ids, :text, :null => false
       t.column :is_public, :boolean, :null => false, :default => true
+      t.column :last_accessed_at, :datetime
       t.timestamps
     end