On one of my forms, a combo box that contains Item_ID and is filtered by another control which contains a combo box of Categories. If nothing in Categories has been selected then the value "0" automatically appears. However, there is no Item with the ID of 0 so if anyone tries to save and exit this form, it should not save since there Item ID is obviously wrong. How can I make it so that it will only accept Item_ID's that are in the tables?