module Enumerable

def index_by

# => { "Chade- Fowlersburg-e" => , "David Heinemeier Hansson" => , ...}
people.index_by { |person| "#{person.first_name} #{person.last_name}" }

# => { "nextangle" => , "chade-" => , ...}
people.index_by(&:login)

element as the value.
Convert an enumerable to a hash, using the block result as the key and the 
def index_by
  if block_given?
    result = {}
    each { |elem| result[yield(elem)] = elem }
    result
  else
    to_enum(:index_by) { size if respond_to?(:size) }
  end
end