Convert an enumerable to a hash

index_by

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

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