gistfile1.rb

# http://apidock.com/rails/Enumerable/group_by#508-Array-clustering
 module Enumerable
    # clumps adjacent elements together
    # >> [2,2,2,3,3,4,2,2,1].cluster{|x| x}
    # => [[2, 2, 2], [3, 3], [4], [2, 2], [1]]
    def cluster
      cluster = []
      each do |element|
        if cluster.last && yield(cluster.last.last) == yield(element)
          cluster.last << element
        else
          cluster << [element]
        end
      end
      cluster
    end
  end

# Document.all.cluster{|document| [document.created_on, document.type]}