JPA: how to composite an entity from columns of multiple tables (not save an entity to mulitple tables)

Question

I have two entities:

@Entity
@Table(name="TableA")
public class TableA {
     @Id
     @Column(name="id")
     long id;

     @Column(name="tableB_id")
     long tbId;

     @Column(name="column1", table="TableB")
     String tbColumn1; 
}

@Entity
@Table(name="TableB")
public class TableB {
     @Id
     @Column(name="id")
     long id;

     @Column(name="column1")
     String column1; 
}

TableA has a foreign key 'tbId' to TableB.id. And TableB has a column named "column1", now I want to get "column1" in TableA entity by some sort of join. What's way I should go in terms of JPA? This is not OneToOne as I don't want to wire entire TableB entity in TableA.

Answer 1

If you want to group columns within one object only for reading data I suggest you two ways:

1. Creating DB VIEW and map it as an Entity.
2. Using Result Classes Constructor Expression, it will instantiate and populate objects of provided class based on results query return.

From ObjectDB on Result Classes Constructor Expression:

JPA supports wrapping JPQL query results with instances of custom result classes. This is mainly useful for queries with multiple SELECT expressions, where custom result objects can provide an object oriented alternative to representing results as Object[] elements.