Doing it in Ruby

1.9.3p448 :007 > Resource.all.sort_by(&:average_rating)
  Resource Load (0.4ms)  SELECT "resources".* FROM "resources"
  Rating Load (0.3ms)  SELECT "ratings".* FROM "ratings" WHERE "ratings"."resource_id" = $1 ORDER BY "ratings"."id" ASC LIMIT 1  [["resource_id", 1]]
  Rating Load (0.2ms)  SELECT "ratings".* FROM "ratings" WHERE "ratings"."resource_id" = $1  [["resource_id", 1]]
  Rating Load (0.2ms)  SELECT "ratings".* FROM "ratings" WHERE "ratings"."resource_id" = $1 ORDER BY "ratings"."id" ASC LIMIT 1  [["resource_id", 2]]
  Rating Load (0.2ms)  SELECT "ratings".* FROM "ratings" WHERE "ratings"."resource_id" = $1  [["resource_id", 2]]
 => [#<Resource id: 2, title: nil, summary: nil, link: nil, created_at: "2013-07-29 13:53:52", updated_at: "2013-07-29 13:53:52", user_id: nil>, #<Resource id: 1, title: nil, summary: nil, link: nil, created_at: "2013-07-29 13:52:36", updated_at: "2013-07-29 13:52:36", user_id: nil>]

gistfile1.rb

class Resource < ActiveRecord::Base
  def self.all_with_rating
    select('resources.id, avg(ratings.value) as avg_rating').
      joins(:ratings).
      order('avg(ratings.value)').
      group('resources.id')
  end
end

Usage

1.9.3p448 :001 > Resource.all
  Resource Load (0.4ms)  SELECT "resources".* FROM "resources"
 => #<ActiveRecord::Relation [#<Resource id: 1, title: nil, summary: nil, link: nil, created_at: "2013-07-29 13:52:36", updated_at: "2013-07-29 13:52:36", user_id: nil>, #<Resource id: 2, title: nil, summary: nil, link: nil, created_at: "2013-07-29 13:53:52", updated_at: "2013-07-29 13:53:52", user_id: nil>]>
1.9.3p448 :002 > Rating.all
  Rating Load (0.3ms)  SELECT "ratings".* FROM "ratings"
 => #<ActiveRecord::Relation [#<Rating id: 1, user_id: nil, resource_id: 1, value: 4, created_at: "2013-07-29 13:52:53", updated_at: "2013-07-29 13:53:11">, #<Rating id: 2, user_id: nil, resource_id: 1, value: 3, created_at: "2013-07-29 13:53:26", updated_at: "2013-07-29 13:53:26">, #<Rating id: 3, user_id: nil, resource_id: 2, value: 2, created_at: "2013-07-29 13:54:02", updated_at: "2013-07-29 13:54:02">, #<Rating id: 4, user_id: nil, resource_id: 2, value: 1, created_at: "2013-07-29 13:54:04", updated_at: "2013-07-29 13:54:04">, #<Rating id: 5, user_id: nil, resource_id: 2, value: 3, created_at: "2013-07-29 13:54:06", updated_at: "2013-07-29 13:54:06">]>
1.9.3p448 :006 > Resource.select('resources.id, avg(ratings.value) as avg_rating').joins(:ratings).order('avg(ratings.value)').group('resources.id').map {|r| [r.id, r.avg_rating.to_s] }
  Resource Load (0.6ms)  SELECT resources.id, avg(ratings.value) as avg_rating FROM "resources" INNER JOIN "ratings" ON "ratings"."resource_id" = "resources"."id" GROUP BY resources.id ORDER BY avg(ratings.value)
 => [[2, "2.0"], [1, "3.5"]]