As per Hibernate documentation, the explanation for @MapsId annotation is given as :
In the embedded id object, the association is represented as the identifier of the associated entity. But you can link its value to a regular association in the entity via the @MapsId annotation. The @MapsId value correspond to the property name of the embedded id object containing the associated entity's identifier. In the database, it means that the Customer.user and the CustomerId.userId properties share the same underlying column (user_fk in this case).
@Entity
class Customer {
@EmbeddedId CustomerId id;
boolean preferredCustomer;
@MapsId("userId")
@JoinColumns({
@JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),
@JoinColumn(name="userlastname_fk", referencedColumnName="lastName")
})
@OneToOne User user;
}
@Embeddable
class CustomerId implements Serializable {
UserId userId;
String customerNumber;
//implements equals and hashCode
}
@Entity
class User {
@EmbeddedId UserId id;
Integer age;
}
@Embeddable
class UserId implements Serializable {
String firstName;
String lastName;
//implements equals and hashCode
}
Also it says:
While not supported in JPA, Hibernate lets you place your association directly in the embedded id component (instead of having to use the @MapsId annotation).
@Entity
class Customer {
@EmbeddedId CustomerId id;
boolean preferredCustomer;
}
@Embeddable
class CustomerId implements Serializable {
@OneToOne
@JoinColumns({
@JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),
@JoinColumn(name="userlastname_fk", referencedColumnName="lastName")
})
User user;
String customerNumber;
//implements equals and hashCode
}
@Entity
class User {
@EmbeddedId UserId id;
Integer age;
}
@Embeddable
class UserId implements Serializable {
String firstName;
String lastName;
//implements equals and hashCode
}
I tried to generate the tables using Hibernate itself (hbm2ddl.auto=create) to understand how the @MapsId annotation is used. Here are my observations:
If my entity declaration for Customer and User are like this:
@Entity
@Table(name="TBL_CUSTOMER")
public class Customer {
@EmbeddedId CustomerId id;
boolean preferredCustomer;
@MapsId("userId")
@JoinColumns({
@JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),
@JoinColumn(name="userlastname_fk", referencedColumnName="lastName")
})
@OneToOne User user;
}
@Entity
@Table(name="TBL_USER")
class User {
@EmbeddedId UserId id;
Integer age;
}
Then the DDL statements generated by Hibernate says:
Hibernate: create table TBL_CUSTOMER (customerNumber varchar2(255 char) not null, preferredCustomer number(1,0) not null, userfirstname_fk varchar2(255 char) not null, userlastname_fk varchar2(255 char) not null, primary key (customerNumber, userfirstname_fk, userlastname_fk))
Hibernate: create table TBL_USER (firstName varchar2(255 char) not null, lastName varchar2(255 char) not null, age number(10,0), primary key (firstName, lastName))
Hibernate: alter table TBL_CUSTOMER add constraint UK_chvh5mukc81xk9t6fis3skab unique (userfirstname_fk, userlastname_fk)
Hibernate: alter table TBL_CUSTOMER add constraint FK_chvh5mukc81xk9t6fis3skab foreign key (userfirstname_fk, userlastname_fk) references TBL_USER
Now if I change my Customer entity to:
@Entity
@Table(name="TBL_CUSTOMER")
public class Customer {
@EmbeddedId CustomerId id;
boolean preferredCustomer;
@OneToOne User user;
}
Then the DDL statements are:
Hibernate: create table TBL_CUSTOMER (customerNumber varchar2(255 char) not null, firstName varchar2(255 char), lastName varchar2(255 char), preferredCustomer number(1,0) not null, user_firstName varchar2(255 char), user_lastName varchar2(255 char), primary key (customerNumber, firstName, lastName))
Hibernate: create table TBL_USER (firstName varchar2(255 char) not null, lastName varchar2(255 char) not null, age number(10,0), primary key (firstName, lastName))
Hibernate: alter table TBL_CUSTOMER add constraint FK_et3bgekef237d4kov7b9oqt85 foreign key (user_firstName, user_lastName) references TBL_USER
In this case, I see 2 extra columns (firstname & lastname) for TBL_CUSTOMER if I remove @MapsId and @JoinColumn annotations. Also an extra alter command is not there in this case.
I am new to Hibernate, so I am finding it difficulty in understanding the explanation given in Hibernate docs, what is the purpose of @MapsId, when we have to use it and how it affects the underlying database schema.
Also I have gone through this SO post - can someone please explain me @MapsId in hibernate? but I am not able to get clear information on this annotation.
