def main_field= field_name
      name = field_name.to_s.alpha_numeric.to_sym
      raise ArgumentException, "Not a valid field name: #{field_name}"  if !valid_field_name?(name)
      @main_field ||= name
    end
def main_field= field_name
      name = field_name.to_s.alpha_numeric.to_sym
      raise ArgumentException, "Not a valid field name: #{field_name}"  if !valid_field_name?(name)
      @main_field ||= name
    end
Set the main field of the behavior