Hibernate计算的集合@Formula

Question

is it possible to have and Entity with a field calculated using Formula when the field is a Collection (let's say it's a Set )?

Here's the dummy example of what I'm trying to achive:

@Formula(value = 
    "SELECT NEW com.example.entity.Person(p.name, p.age) FROM Person p")
lateinit var people :Set<Person>

Answer 1

From the JavaDoc for @Formula :

Defines a formula (derived value) which is a SQL fragment ...

You have to think of the fragment you write as an replacement in the select statement:

SELECT (formulaValue) AS propertyName FROM ....

Everything you can write into formulaValue can be used in @Formula .

Your example is not a valid SQL fragment and as you can see, it is not possible to return more than one value from a @Formula .

But you could use @Subselect and a wrapper object instead:

@Entity
@Subselect("SELECT name, age FROM Person p")
public class PersonWrapper {

  @Id
  private String name;

  private int age;

}

(in Java, as I'm not aware of the correct syntax of Kotlin)

If you really need the collection of these values in another entity, you need something to join on (which is missing in your example) and use that in an @OneToMany or @ManyToMany . Otherwise there is no use to have all these values in your entity, as all entities would have the same collection.