Fix bug.

lib/bulletin_model/cache.rb

@@ -8,8 +8,10 @@ module BulletinModel
       before_destroy :cache_tag_ids
     end
     def cache_tag_ids
-      @tag_ids = self.tag_ids
-      @org_tag_ids = self.org_tag_ids
+      if self.class == Bulletin
+        @tag_ids = self.tag_ids
+        @org_tag_ids = self.org_tag_ids
+      end
     end
     def do_before_save
       if self.class == SubPart