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javahibernatejpaormhibernate-mapping

How to map calculated properties with JPA and Hibernate

My Java bean has a childCount property. This property is not mapped to a database column. Instead, it should be calculated by the database with a COUNT() function operating on the join of my Java bean and its children. It would be even better if this property could be calculated on demand / "lazily", but this is not mandatory.

In the worst case scenario, I can set this bean's property with HQL or the Criteria API, but I would prefer not to.

The Hibernate @Formula annotation may help, but I could barely find any documentation.

Any help greatly appreciated. Thanks.

Solution

  • JPA doesn't offer any support for derived property so you'll have to use a provider specific extension. As you mentioned, @Formula is perfect for this when using Hibernate. You can use an SQL fragment:

    @Formula("PRICE*1.155")
private float finalPrice;

    Or even complex queries on other tables:

    @Formula("(select min(o.creation_date) from Orders o where o.customer_id = id)")
private Date firstOrderDate;

    Where id is the id of the current entity.

    The following blog post is worth the read: Hibernate Derived Properties - Performance and Portability.

    Without more details, I can't give a more precise answer but the above link should be helpful.

    See also: