How-to: DELETE

Example 1

Task

We have an order with a given date and buyer and also with lines that refer to the books.

CLASS Order 'Order';

CLASS Book 'Book';
name 'Name' = DATA ISTRING[50] (Book);

CLASS OrderDetail 'Order line';
order 'Order' = DATA Order (OrderDetail) NONULL DELETE;
book 'Book' = DATA Book (OrderDetail);
nameBook 'Book' (OrderDetail d) = name(book(d));

We need to create an action that deletes the book for which no orders have been placed.

Solution

delete (Book b)  {
    IF NOT [ GROUP SUM 1 BY book(OrderDetail d)](b) THEN
        DELETE b;
    ELSE
        MESSAGE 'It is forbidden to delete a book, as there is an order for it';
}

Example 2

Task

Similar to Example 1.

We need to create an action that clears the order by deleting all its lines.

Solution

clear (Order o)  {

    // Option 1
    DELETE OrderDetail d WHERE order(d) == o;

    // Option 2
    FOR order(OrderDetail d) == o DO
        DELETE d;
}

Both these implementations are acting similarly.