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Spring data JPA and inheritance

05 Apr 2016

Let’s start adding user management to our application. There are two kind of personal information that we want to capture at the moment: existing users and invitations sent out for new potential customers. Invitations and Users have a lot in common so let’s start with entities for our Spring Boot application

@Entity
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public abstract class Person {

    private @Id @GeneratedValue(strategy = GenerationType.TABLE) Long id;
    private String firstName;
    private String lastName;
    private String email;

    ...
}

@Entity
public class Invitation extends Person {

    ...

}

@Entity
public class User extends Person {

    ...

}

Let’s start with keeping invitations and users in completely separate tables by using InheritanceType.TABLE_PER_CLASS

To work with the set of data create a simple repository

public interface PersonRepository extends JpaRepository<Person, Long> {

    Page<Person> findAll(Pageable pageable);
}

If we now issue a simple Pageable request

Pageable pageable = new PageRequest(0, 10, Sort.Direction.ASC, "firstName", "lastName");
Page<Person> firstPage = personRepository.findAll(pageable);
for (Person person : firstPage) {
    System.out.println(person);
}

So let’s see what kind of queries the database issues in the background by placing the following application.properties in /src/test/resources

spring.jpa.show-sql = true
spring.jpa.properties.hibernate.format_sql = true

logging.level.org.hibernate.SQL = DEBUG
logging.level.org.hibernate.type.descriptor.sql.BasicBinder = TRACE

If we stay with TABLE_PER_CLASS strategy the hibernate issues the following query

SELECT person0_.id AS id1_1_,
       person0_.email AS email2_1_,
       person0_.first_name AS first_na3_1_,
       person0_.last_name AS last_nam4_1_,
       person0_.date AS date1_0_,
       person0_.clazz_ AS clazz_
  FROM ( SELECT id,
                email,
                first_name,
                last_name,
                null AS date,
                1 AS clazz_
           FROM user UNION ALL
         SELECT id,
                email,
                first_name,
                last_name,
                date,
                2 AS clazz_
           FROM invitation
       ) person0_
 ORDER BY person0_.first_name ASC,
          person0_.last_name ASC
 LIMIT ?

For SINGLE_TABLE strategy we get

SELECT person0_.id AS id2_0_,
       person0_.email AS email3_0_,
       person0_.first_name AS first_na4_0_,
       person0_.last_name AS last_nam5_0_,
       person0_.date AS date6_0_,
       person0_.dtype AS dtype1_0_
  FROM person person0_
 ORDER BY person0_.first_name ASC,
          person0_.last_name ASC
 LIMIT ?

And for the JOINED strategy we get

SELECT person0_.id AS id1_1_,
       person0_.email AS email2_1_,
       person0_.first_name AS first_na3_1_,
       person0_.last_name AS last_nam4_1_,
       person0_2_.date AS date1_0_,
       CASE
           WHEN person0_1_.id IS NOT NULL THEN 1
           WHEN person0_2_.id IS NOT NULL THEN 2
           WHEN person0_.id IS NOT NULL THEN 0
       END as clazz_
  FROM person person0_
  LEFT OUTER JOIN user person0_1_
               ON person0_.id=person0_1_.id
  LEFT OUTER JOIN invitation person0_2_
               ON person0_.id=person0_2_.id
 ORDER BY person0_.first_name ASC,
          person0_.last_name ASC
 LIMIT ?

That’s it for now

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