Question

This has bugged me for years but I cannot find anything about it in jpa references nor this site. When an entity has a computed member (Inverse via mappedby, @Formula, …), there doesn’t seem to be any way to get a transactionally consistent update of that computed member. The pattern seems to be that the setter of the one side needs to know about the consumers and update them in situ. I must be missing something. Hopefully the below is illustrative enough.

I wouldn’t mind if the requirement was that I had to call em.getReference or .find again on the obj w/ the computed members just in case the ones from the original fetch are out of sync, but that just returns the exact same instance w/ no updates of the fields.

import java.util.Set;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.EntityManager;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.persistence.OneToMany;
import javax.persistence.PersistenceContext;

import org.springframework.transaction.annotation.Transactional;

@Entity
public class InverseProblemDto {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private int id;
    @OneToMany(mappedBy="problem")
    public Set<OwnerDto> owners;

    public int otherField = 0;
}

@Entity
public class OwnerDto {
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id")
    private int id;

    @ManyToOne
    public InverseProblemDto problem; 

    public int yetAnotherField = 0;
}


@Transactional
public void wontWork(int dbId) {
    @PersistenceContext
    EntityManager em;

    InverseProblemDto probDto = em.find(InverseProblemDto.class, dbId);
    probDto.otherField++;

    for (OwnerDto other : probDto.owners) {
        // do something
    }

    OwnerDto newOwner = new OwnerDto();
    newOwner.problem = probDto;
    em.persist(newOwner);

    // do more

    // How to ensure this finds newOwner w/o breaking transactional integrity by forcing a flush and refresh???
    for (OwnerDto other : probDto.owners) {
        if (other.id == newOwner.id) System.out.println("Yeah! found it");
    }

}

Answer 1

Flush and refresh do not commit the transaction but only issue the pending sql to the database.

You could always change the value yourself like this :

@Entity
public class InverseProblemDto {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private int id;
    @OneToMany(mappedBy="problem")
    public Set<OwnerDto> owners;

    public int otherField = 0;

    public void addOwner(OwnerDto owner) {
        ...
    }
}

@Entity
    public class OwnerDto {
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id")
    private int id;

    @ManyToOne
    public InverseProblemDto problem; 

    public int yetAnotherField = 0;

    public void setProblem(InverseProblemDto problem) {
        if(problem != null) {
            problem.addOwners(this);
        }
        this.problem = problem;
    }

}