Avoid caching sorted id's, just use relation.key.

lib/mongoid/sorted_relations.rb

@@ -8,19 +8,17 @@ def self.included(base)
 
     def freeze_relation_ids
       @cache_sorted_documents = {}
-      @cache_sorted_ids = {}
 
       self.relations.each do |k,v|
         if [:references_many, :references_and_referenced_in_many].include? v.macro
-          @cache_sorted_ids[v.key] = [self.send(v.key)].flatten.map{ |rid| rid.to_s }
           self.class.send(:define_method, "sorted_#{v.name}") { sorted_relation v }
         end
       end
     end
 
     def sorted_relation(relation)
       if not @cache_sorted_documents[relation.name]
-        documents = self.send(relation.name).sort_by { |x| @cache_sorted_ids[relation.key].index(x.id.to_s) }
+        documents = self.send(relation.name).sort_by { |x| self.send(relation.key).index(x.id) }
         @cache_sorted_documents[relation.name] = Mongoid::FakeCriteria.new(documents)
       end
 

spec/mongoid/sorted_relations_spec.rb

@@ -9,6 +9,21 @@
       before { book.save and book.reload }
 
       the("book.sorted_authors.map(&:name)") { should eql ['Sun Tzu', 'Sun Wu', 'Lao Zi'] }
+
     end
   end
+
+  context "modifying a loaded model" do
+    let (:book) {Book.new(:title => 'The Art of War')}
+
+    it "enables sorting on a preloaded model" do
+      author1 = Author.create(:name => 'auth1')
+      author2 = Author.create(:name => 'auth2')
+      book.authors << author2
+      book.authors << author1
+      book.sorted_authors.map(&:name).should eql ['auth2', 'auth1']
+    end
+
+  end
+
 end