使用group_by方法，如下所示

students = [
  {name: "James", cohort: :December},
  {name: "David", cohort: :April},
  {name: "Kyle", cohort: :December}
]

grouped_students = students.group_by { |student| student[:cohort] }

grouped_students.each do |cohort, students|
  puts "#{cohort} = #{students.map { |student| student[:name] }.join(', ')}"
end

下面是另一种使用each_with_object方法的方法。

students_by_cohort = students.each_with_object({}) do |student, result|
  cohort = student[:cohort]
  result[cohort] ||= []
  result[cohort] << student[:name]
end

students_by_cohort.each do |cohort, students|
  puts "#{cohort}: #{students.join(", ")}"
end

输出

December = James, Kyle
April = David