タグを木構造に

Gemfile

@@ -71,3 +71,6 @@ group :production do
   gem 'rails_12factor'
   gem 'pg'
 end
+
+# tree structure
+gem 'ancestry'

Gemfile.lock

@@ -32,6 +32,8 @@ GEM
       multi_json (~> 1.3)
       thread_safe (~> 0.1)
       tzinfo (~> 0.3.37)
+    ancestry (2.0.0)
+      activerecord (>= 3.0.0)
     arel (4.0.1)
     atomic (1.1.14)
     bcrypt-ruby (3.1.2)
@@ -161,6 +163,7 @@ PLATFORMS
   ruby
 
 DEPENDENCIES
+  ancestry
   better_errors
   binding_of_caller
   coderay

db/migrate/20131213005609_add_ancestry_to_tags.rb

+class AddAncestryToTags < ActiveRecord::Migration
+  def change
+    add_column :tags, :ancestry, :string
+  end
+end