Greeting all: Thank you for all help. I just get some troubles that I misunderstand something. My document about booking a hotel� room, that a field was named �Customer holding this record�. The comment said that: The id value of the customer who has booked this. If this field is all blanks, the record is available for sale. If I want to coding create method that either I creating a new record to the database or update the �Customer holding this record� field, it�s mean some customer booking this room. Well, how can I choice it, dear friends, please help me.