def all_roots_valid?
      if acts_as_nested_set_options[:scope]
        roots.group_by{|record| scope_field_names.collect{|field| record.send(field.to_sym)}}.all? do |scope, grouped_roots|
          each_root_valid?(grouped_roots)
        end
      else
        each_root_valid?(roots)
      end
    end
def all_roots_valid?
      if acts_as_nested_set_options[:scope]
        roots.group_by{|record| scope_field_names.collect{|field| record.send(field.to_sym)}}.all? do |scope, grouped_roots|
          each_root_valid?(grouped_roots)
        end
      else
        each_root_valid?(roots)
      end
    end
Wrapper for each_root_valid? that can deal with scope
 Warning: Very expensive!  Do not use unless you know what you are doing.