From: Martin DeMello <martindemello@...>
Date: 2008-08-12T02:35:07+09:00
Subject: Re: Sort array by two attributes? (like sql "order by A, B")

On Mon, Aug 11, 2008 at 8:48 AM, Erik Veenstra <erikveen@gmail.com> wrote:
>> That's a neat trick, thanks!  I actually like the first one better
>> though as you can swap a & b around in each test to get (eg) sorted by
>> name ascending and then a date field descending.
>
> Just use -x.b:
>
> a.sort_by{|x| [x.name, -x.date]}

Doesn't always work. I keep this handy:

class RevCmp
    attr_reader :this

    def initialize(obj)
      @this = obj
    end

    def <=>(other)
      other.this <=> @this
    end

    # not delegating anything else because this is explicitly a throwaway
    # object used only inside a sort_by block
end

and then you have

  a.sort_by {|x| [x.name, RevCmp.new(x.date)]}

you could even use a top-level method so you can say

  a.sort_by {|x| [x.name, descending(x.date)]}

where descending(x) returns RevCmp.new(x)

You could even mix it into object to get

  a.sort_by {|x| [x.name, x.date._descending_]}

where I use the underscores as a cosmetic way of making it stand out
inside the sort block

martin