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.
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.