こんなの便利だなあって思った

each_of.rb

module Enumerable
  def each_of key, &blk
    return Enumerator.new self, :each_of, key unless block_given?
    each do |e|
      blk.call e[key]
    end
  end
end

res = [[1, 2], [3, 4]].each_of(0).map do |i|
  puts i
  i
end

p res
# 1
# 3
# [1, 3]

res = [{a: 1, b: 2}, {a: 3, c: 4}].each_of(:a).map do |i|
  puts i
  i
end

p res
# 1
# 3
# [1, 3]