def fields=(fields)
      @fields = fields.collect do |field|
        r = field.intern
        raise ArgumentError.new(_("Cannot have fields named %{name}") % { name: r }) if INVALID_FIELDS.include?(r)
        r
      end
    end
def fields=(fields)
      @fields = fields.collect do |field|
        r = field.intern
        raise ArgumentError.new(_("Cannot have fields named %{name}") % { name: r }) if INVALID_FIELDS.include?(r)
        r
      end
    end
Customize this so we can do a bit of validation.