can someone please explain me @MapsId in hibernate?

Question

Can someone please explain to me @MapsId in hibernate? I\'m having a hard time understanding it.

It would be great if one could explain it with an examp

Answer 1

IMHO, the best way to think about @MapsId is when you need to map a composite key in a n:m entity.

For instance, a customer can have one or more consultant and a consultant can have one or more customer:

And your entites would be something like this (pseudo Java code):

@Entity
public class Customer {
   @Id
   private Integer id;

   private String name;
}

@Entity
public class Consultant {
   @Id
   private Integer id;

   private String name;

   @OneToMany
   private List customerByConsultants = new ArrayList<>();

   public void add(CustomerByConsultant cbc) {
      cbc.setConsultant(this);
      this.customerByConsultant.add(cbc);
   }
}

@Embeddable
public class ConsultantByConsultantPk implements Serializable {

    private Integer customerId;

    private Integer consultantId;
}

@Entity
public class ConsultantByConsultant {

   @EmbeddedId
   private ConsultantByConsultantPk id = new ConsultantByConsultantPk();

   @MapsId("customerId")
   @JoinColumn(insertable = false, updatable = false)
   Customer customer;

   @MapsId("consultantId")
   @JoinColumn(insertable = false, updatable = false)
   Consultant consultant;
}

Mapping this way, JPA automagically inserts Customer and Consultant ids in the EmbeddableId whenever you save a consultant. So you don't need to manually create the ConsultantByConsultantPk.