Enumerable#group_by_first and Enumerable#group_by_last do in contrast du Enumerable#group_by not return an Array on the value side of the hash but only a single element.

enumerable_group_by_extension.rb

module Enumerable
  def group_by_first
    inject({}) do |hash, element|
      key = yield(element)
      hash[key] ||= element
      hash
    end
  end

  def group_by_last(&block)
    reverse.group_by_first(&block)
  end
end