我会这样做：

todos = [
  ["Send invoice", "money"],
  ["Clean room", "organize"],
  ["Pay rent", "money"],
  ["Arrange books", "organize"],
  ["Pay taxes", "money"],
  ["Buy groceries", "food"]
]

todos.group_by(&:last).map { |key, values| [key, values.map(&:first)] }

# => [["money", ["Send invoice", "Pay rent", "Pay taxes"]], ["organize", ["Clean room", "Arrange books"]], ["food", ["Buy groceries"]]]

这是怎么回事？Enumerable#group_by返回按其Array#last值分组的嵌套数组，并将返回如下所示的嵌套散列结构：

{
  "money"=>[["Send invoice", "money"], ["Pay rent", "money"], ["Pay taxes", "money"]],
  "organize"=>[["Clean room", "organize"], ["Arrange books", "organize"]],
  "food"=>[["Buy groceries", "food"]]
}

在方法链的下一步中，它使用Enumerable#map将每个键/值对转换为最终结构，方法是返回一个包含来自grouped_by的键的数组和一个仅包含来自原始值的first值的嵌套数组。例如，它将在一次迭代中接受此输入，并将其转换为：

{ "organize"=>[["Clean room", "organize"], ["Arrange books", "organize"]] }

# with `key` being "organize", and `values` being the nested arrays on the right

# getting translated into:

# => ["organize", ["Clean room", "Arrange books"]]

我们可以使用#each_with_object在todos上迭代，构建一个散列，对于这个任务来说，这似乎是一个更有用的数据结构。

h = todos.each_with_object({}) do |arr, h|
  task, category = arr 
  h[category] ||= []
  h[category] << task 
end

# => {"money"=>["Send invoice", "Pay rent", "Pay taxes"],

# "organize"=>["Clean room", "Arrange books"],

# "food"=>["Buy groceries"]}

这样做相当于：

h = {}

for arr in todos
  task = arr.first
  category = arr.last

  h[category] = [] unless h.has_key? category
  h[category].push(task)
end

要把它变成数组的数组。

h.map { |k, v| [k, v] }

# => [["money", ["Send invoice", "Pay rent", "Pay taxes"]],

# ["organize", ["Clean room", "Arrange books"]],

# ["food", ["Buy groceries"]]]

todos = [
  ["Send invoice", "money"],
  ["Clean room", "organize"],
  ["Pay rent", "money"],
  ["Arrange books", "organize"],
  ["Pay taxes", "money"],
  ["Buy groceries", "food"]
]

todos.group_by(&:last).transform_values{ |v| v.map(:first) }

在这里，我们使用带有速记方法调用的group_by方法，这将返回一个散列，其中键是块的计算结果，值是集合中与键对应的元素数组。
我们还对块使用transform_values方法，并根据给定的块对哈希值进行转换
上述陈述的结果

=> {"money"=>["Send invoice", "Pay rent", "Pay taxes"], "organize"=>["Clean room", "Arrange books"], "food"=>["Buy groceries"]}