to_json
railsの配列を簡単にjsonにしてくれる便利メソッド
{"word":<%= @word.to_json(:only => [:id, :title]) %>
"parents":<%= @word.parents.map{|w|w.from}.to_json(:only => [:id, :title]) %>
"children":<%= @word.children.map{|w|w.to}.to_json(:only => [:id, :title]) %>
}