lib/mongoid_nested_set/validation.rb
def no_duplicates_for_fields?
      roots.group_by{|record| scope_field_names.collect{|field| record.send(field.to_sym)}}.all? do |scope, grouped_roots|
        [left_field_name, right_field_name].all? do |field|
          grouped_roots.first.nested_set_scope.only(field).group_by {|doc| doc.send(field)}.all? {|k, v| v.size == 1}
        end
      end
    end
def no_duplicates_for_fields?
      roots.group_by{|record| scope_field_names.collect{|field| record.send(field.to_sym)}}.all? do |scope, grouped_roots|
        [left_field_name, right_field_name].all? do |field|
          grouped_roots.first.nested_set_scope.only(field).group_by {|doc| doc.send(field)}.all? {|k, v| v.size == 1}
        end
      end
    end
Warning: Very expensive!  Do not use unless you know what you are doing.