example.rb

require 'active_support'
# in some cases i've found JSON.pretty_generate doesn't output properly
# when combined with ActiveSupport::OrderedHash without json/pure
# require 'json/pure'

def returning(value)
  yield(value)
  value
end

def convert_hash_to_ordered_hash_and_sort(object, deep = false)
# from http://seb.box.re/2010/1/15/deep-hash-ordering-with-ruby-1-8/
  if object.is_a?(Hash)
    # Hash is ordered in Ruby 1.9! 
    res = returning(RUBY_VERSION >= '1.9' ? Hash.new : ActiveSupport::OrderedHash.new) do |map|
      object.each {|k, v| map[k] = deep ? convert_hash_to_ordered_hash_and_sort(v, deep) : v }
    end
    return res.class[res.sort {|a, b| a[0].to_s <=> b[0].to_s } ]
  elsif deep && object.is_a?(Array)
    array = Array.new
    object.each_with_index {|v, i| array[i] = convert_hash_to_ordered_hash_and_sort(v, deep) }
    return array
  else
    return object
  end
end

myhsh = {'x' => {3 => nil, 1 => nil, 5 => nil}, 'a' => {3 => nil, 1 => nil, 5 => nil}, 'a2' => {3 => nil, 1 => nil, 5 => nil}}

myhsh = convert_hash_to_ordered_hash_and_sort(myhsh, true)

puts JSON.pretty_generate(myhsh)

output.json

{
  "a": {
    "1": null,
    "3": null,
    "5": null
  },
  "a2": {
    "1": null,
    "3": null,
    "5": null
  },
  "x": {
    "1": null,
    "3": null,
    "5": null
  }
}