JPA Repository with Inheritance - SQL Error: Column 'clazz_' not found

Question

I have designed the entities in my application to follow the Hibernate's inheritance strategy Inheritance.JOINED.

The base abstract class is Activity and the concrete derived classes are Exercise and Contest: (currently, Exercise and Contest are almost identical)

Activity.java

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
@Table(name = "activity")
@Data
@NoArgsConstructor
@AllArgsConstructor
public abstract class Activity {
    
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "activity_id")
    private Integer id;
    
    @Column(name = "name", nullable = false)
    private String name;
    
    @Column(name = "details")
    private String details;
    
    @ManyToOne
    @JoinColumn(name="course_id", nullable = false)
    private Course course;
}

Exercise.java

@Entity
@Table(name = "exercise")
@Data
@EqualsAndHashCode(callSuper = true)
@NoArgsConstructor
@AllArgsConstructor
public class Exercise extends Activity {
    
    @Column(name = "deadline")
    private Date deadline;
    
    @Column(name = "task", nullable = false)
    private String task;
}

Contest.java

@Entity
@Table(name = "contest")
@Data
@EqualsAndHashCode(callSuper = true)
@NoArgsConstructor
@AllArgsConstructor
public class Contest extends Activity {
    
    @Column(name = "start")
    private Date start;
    
    @Column(name = "task", nullable = false)
    private String task;
}

My Repository looks like this:

ActivityRepository.java

public interface ActivityRepository extends JpaRepository<Activity, Integer>{
    
    @Query(value = "SELECT * FROM activity a WHERE course_id=:course_id", nativeQuery = true)
    public List<Activity> getByCourseId(@Param("course_id") int course_id);
}

When executing the Query getByCourseId(), I receive the SQL Error Column 'clazz_' not found.

Answer 1

Don't use a native query. Try this :

public List<Activity> findByCourse(Course course);

It should work without writing the query which is :

select a from Activity where a.course = :course