How to assign several fields as a primary key of an entity (using JPA) [duplicate]

Question

One can assign a primary key for its class by using @Id annotation in JPA. My question is what if one doesn't want to have an auto generated key in his tables and use fields (maybe more than one) as a primary key.

Let's say we have a person table with SSN, NATIONALITY and NAME. SSN is defined as the number a person is identified by in his country. Thus, we might have two persons with the same number in two different countries. The primary key for this table can be SSN+NATIONALITY. Is there any way to map these two fields using JPA and map it to an object? or the only way it to create an auto generated id and use @Id annotation

CREATE TABLE PERSON (
     SSN   INT,
     NATIONALITY VARCHAR,
     NAME VARCHAR
  )

Answer 1

Yes, it is possible. A composite primary key consist of multiple primary key fields. Each primary key field must be one of the supported by JPA type. For your table, you have:

@Entity @IdClass(PersonId.class)
public class Person {
    @Id int ssn;
    @Id String nationality;
     ....
}

For entity with multiple key, JPA requires defining a special ID class. This class should be attached to the entity class using the @IdClass annotation.

class PersonId {
    int ssn;
    String nationality;
}

The ID class reflects the primary key fields and its objects can represent primary key values