diff --git a/Gemfile b/Gemfile
index 55030971d..623179534 100644
--- a/Gemfile
+++ b/Gemfile
@@ -49,6 +49,9 @@ gem 'rqrcode_png'
 
 gem 'acts-as-taggable-on', '~> 6.0'
 
+# a tree structure
+gem 'ancestry'
+
 group :development, :test do
   gem 'rspec-rails', '~> 3.8'
 end