From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2008-08-12T00:27:37+09:00
Subject: Re: Sort array by two attributes? (like sql "order by A, B")

On Mon, Aug 11, 2008 at 5:16 PM, Max Williams
<toastkid.williams@gmail.com> wrote:
> IN sql we can pass two arguments to the 'order by' component, and it
> will order the results by A, and then B in the cases where A is the
> same.
>
> Can anyone think of a way to do the same thing with a ruby array?  For
> example, if the array holds objects that have attributes/methods
> "lastname" & "firstname", to order the objects in a similar way to the
> sql query?
>

Here's one way: the trick is to create an array with the fields you want
to order by in the sort_by block:

irb(main):001:0> class A
irb(main):002:1> attr_accessor :a,:b
irb(main):003:1> def initialize a,b
irb(main):004:2> @a = a
irb(main):005:2> @b = b
irb(main):006:2> end
irb(main):007:1> end
=> nil
irb(main):008:0> ary = [A.new(1,2), A.new(1,3), A.new(1,1),
A.new(2,3), A.new(2,1)]
=> [#<A:0xb7b68890 @b=2, @a=1>, #<A:0xb7b6887c @b=3, @a=1>,
#<A:0xb7b68868 @b=1, @a=1>, #<A:0xb7b68854 @b=3, @a=2>, #<A:0xb7b68840
@b=1, @a=2>]
irb(main):009:0> ary.sort_by {|x| [x.a,x.b]}
=> [#<A:0xb7b68868 @b=1, @a=1>, #<A:0xb7b68890 @b=2, @a=1>,
#<A:0xb7b6887c @b=3, @a=1>, #<A:0xb7b68840 @b=1, @a=2>, #<A:0xb7b68854
@b=3, @a=2>]

Hope this helps,

Jesus.