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 example and in what kind of use cases is it most applicable?

Dictate

Even the example provided in @MapsId source code is confusing – Leith

Answer 1

Here is a nice explanation from Object DB.

Designates a ManyToOne or OneToOne relationship attribute that provides the mapping for an EmbeddedId primary key, an attribute within an EmbeddedId primary key, or a simple primary key of the parent entity. The value element specifies the attribute within a composite key to which the relationship attribute corresponds. If the entity's primary key is of the same Java type as the primary key of the entity referenced by the relationship, the value attribute is not specified.

// parent entity has simple primary key

@Entity
public class Employee {
   @Id long empId;
   String name;
   ...
} 

// dependent entity uses EmbeddedId for composite key

@Embeddable
public class DependentId {
   String name;
   long empid;   // corresponds to primary key type of Employee
}

@Entity
public class Dependent {
   @EmbeddedId DependentId id;
    ...
   @MapsId("empid")  //  maps the empid attribute of embedded id
   @ManyToOne Employee emp;
}

Read the API Docs here.

Suasion

But what's so nice about it? Even without @MapsId, one can use JoinColumn to same effect, can he not? And if so, this example does not really tell what's this annotation really good for. – Polydactyl

Since both entities will be sharing the same primary key, the entity with a column designated by @MapsId will in the persistence layer (database) only have primary key column. The idea is to share the primary key between the two entities. – Buehrer

What is an EmbeddedId primary key? How is it different from normal Primary Key? – Rolo

"The value element specifies the attribute within a composite key to which the relationship attribute corresponds. " 1) What is meant by value element, please give an example? 2) What is a composite key? 3) What is the relationship attribute and give an example? – Rolo

@MaksimGumerov it's efficient because you can fetch Dependent just by knowing identifier of Employee. – Impel

"just by knowing identifier of Employee" - how so? There's ManyToOne, thus many Dependent may be associated with the same Employee. – Polydactyl

"The idea is to share the primary key between the two entities" - but can't we just use derived identifiers? I.e. write DependentId not like this, but like "... Employee emp"? No need to use MapsId then. And anyway, I am not saying the approach is inherently bad, I am saying that the cited explanation isn't particularly "nice" :) – Polydactyl

I have similar scenario @Suasion , This mapping worked out for me but how do we save Parent and chil entity in single go with JPA repository. For example I want to save Employee with its list of dependents just by employeeRepo.save(employee) method. Challenge I am facing is what do we set for employeeId attribute of dependent table even before saving .. please assist, have been struck from 3 days – Ecclesiastical

Answer 2

I found this note also useful: @MapsId in hibernate annotation maps a column with another table's column.

It can be used also to share the same primary key between 2 tables.

Example:

@Entity
@Table(name = "TRANSACTION_CANCEL")
public class CancelledTransaction {
    @Id
    private Long id; // the value in this pk will be the same as the
                     // transaction line from transaction table to which 
                     // this cancelled transaction is related

    @OneToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "ID_TRANSACTION", nullable = false)
    @MapsId
    private Transaction transaction;
    ....
}

@Entity
@Table(name = "TRANSACTION")
@SequenceGenerator(name = "SQ_TRAN_ID", sequenceName = "SQ_TRAN_ID")
public class Transaction  {
    @Id
    @GeneratedValue(generator = "SQ_TRAN_ID", strategy = GenerationType.SEQUENCE)
    @Column(name = "ID_TRANSACTION", nullable = false)
    private Long id;
    ...
}

Elysium

Where do I put @MapsId in a bidirectional association? Should both classes have @MapsId. Does it even make a difference? – Biquarterly

I think one table will be the "owner" of the original pk (the one with @Id and @GeneratedValue and @Column) and have a @OneToOne and @JoinColumn with the other table, and the other table will have the @MapsId . However this probably wouldn't work if you wanted to INSERT into the 'other table' first. – Elysium

Nice article about this way of usage (using id from another table as id of other entity) is here vladmihalcea.com/… – Apennines

But if you want filter out transactions from canceled ones.. which is a common cause. How is this more efficient? I mean for SQL you just have to say NOT NULL for TRANSACTION.fk_cancelled_id but in this case, will be more operations. – Kassey

In this specific case here, it is common to use a column like 'type' in the base class and table, where you'd identify if the transaction is of type 'cancel' or other. – Elysium

Ah, found another useful blog post about this subject baeldung.com/hibernate-inheritance – Elysium

Answer 3

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<CustomerByConsultant> customerByConsultants = new ArrayList<>();

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

@Embeddable
public class CustomerByConsultantPk implements Serializable {
    
    private Integer customerId;

    private Integer consultantId;
}

@Entity
public class CustomerByConsultant{
   
   @EmbeddedId
   private CustomerByConsultantPk id = new CustomerByConsultantPk();
   
   @MapsId("customerId")
   @JoinColumn(insertable = false, updatable = false)
   private Customer customer;

   @MapsId("consultantId")
   @JoinColumn(insertable = false, updatable = false)
   private 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 CustomerByConsultantPk.

Gummosis

Answer 4

As he explained Vladimir in his tutorial, The best way to map a @OneToOne relationship is to use @MapsId. This way, you don’t even need a bidirectional association since you can always fetch the Child entity by using the Parent entity identifier.

Bathtub

Answer 5

MapsId lets you use the same primary key between two different entities/tables. Note: when you use MapsId, the CASCADE.ALL flag becomes useless, and you will need to make sure that your entities are saved manually.

Quadrinomial

and removed manually as well – Wheelhorse

Answer 6

In the answer to my question about

Saving an entity with @ManyToOne JPA Composite Key used as Ids adds an extra parameter

It shows another use of @MapsId to solve a mapping problem and what happens if you don't apply @MapsId.

Zinnia