JPA - Setting entity class property from calculated column?

Question

I\'m just getting to grips with JPA in a simple Java web app running on Glassfish 3 (Persistence provider is EclipseLink). So far, I\'m really liking it (bugs in netbeans/gl

Answer 1

There are probably no good ways to do it, only manually:

Object[] r = (Object[]) em.createNativeQuery(
    "select id,title,shorttitle,datestamp,body,true as published, ts_headline(body,q,'ShortWord=0') as headline, type from articles,to_tsquery('english',?) as q where idxfti @@ q order by ts_rank(idxfti,q) desc","ArticleWithHeadline")
    .setParameter(...).getSingleResult();

Article a = (Article) r[0];
a.setHeadline((String) r[1]);

-

@Entity
@SqlResultSetMapping(
    name = "ArticleWithHeadline",
    entities = @EntityResult(entityClass = Article.class),
    columns = @ColumnResult(name = "HEADLINE"))
public class Article {
    @Transient
    private String headline;
    ...
}

Answer 2

AFAIK, JPA doesn't offer standardized support for calculated attributes. With Hibernate, one would use a Formula but EclipseLink doesn't have a direct equivalent. James Sutherland made some suggestions in Re: Virtual columns (@Formula of Hibernate) though:

There is no direct equivalent (please log an enhancement), but depending on what you want to do, there are ways to accomplish the same thing.

EclipseLink defines a TransformationMapping which can map a computed value from multiple field values, or access the database.

You can override the SQL for any CRUD operation for a class using its descriptor's DescriptorQueryManager.

You could define a VIEW on your database that performs the function and map your Entity to the view instead of the table.

You can also perform minor translations using Converters or property get/set methods.

Also have a look at the enhancement request that has a solution using a DescriptorEventListener in the comments.

All this is non standard JPA of course.