>> p = GridParameter.find( :first )
>> GridParameter.natural_key
=> ["grib_edition", "grid_parameter_table_id", "grid_category_id", "grid_parameter_number", "leads_parameter_id"]
>> p.natural_key_values
=> [2, -1, 690, 0, 381]
>> p.natural_key
=> {"grid_parameter_table_id"=>-1, "grid_parameter_number"=>0, "grid_category_id"=>690, "grib_edition"=>2, "leads_parameter_id"=>381}