array#to_tree

to_tree.rb

class Array
  def to_tree(*key_methods,&block)
    reduce({}) do |tree,e|
      key_path = block_given? ? yield(e) : key_methods.map{|m| e.instance_eval(m.to_s)}
      sub_tree = tree
      key_path.each_with_index do |k,i|
        if i == key_path.size - 1
          (sub_tree[k] ||= []) << e
        elsif
          sub_tree = (sub_tree[k] ||= {})
        end
      end
      tree
    end
  end
end
#For example: Person.all.to_tree(:department, :job_title)