Hibernate: Computed property value

Many times you want to have computed value from db as a property value of an entity. For simple computation, where you additional information from DB is not needed, you use java code which is fine. But if you need information from DB, it means additional queries. Hibernate provides a powerful feature known as as derived properties

For example consider there is are table called Order and OrderItem; an Order has multiple OrderItem(s) and OrderItem has a field called price. Now if you want orderPrice property in Order Entity such that
orderPrice = sum of all OrderItem.price*OrderItem.qty where OrderItem.orderId=<my order id>.
This can be done simply by defining a property in hbm file like this .

< property name="orderPrice"
formula="( SELECT SUM (OI.QTY*OI.PRICE) FROM ORDER_ITEM OI OI.ORDER_ID = ID )"/>

These properties are by definition read-only, and the value is computed at load time. You declare the computation as a SQL expression, this translates to a SELECT clause subquery in the SQL query that loads an instance. Make sure you use SQL names (TABLE and column names) and not the Java names (Object and property names). also don't forget to put the formula inside "(" and ")".

Hibernate Reverse engineering tool

May time you want to reverse engineer the Hibernate definitions from already existing scheme. Hibernate tools with Ecliplse plutgin is a very useful for this purpose, more information here on hibernate.org.