Spring Data JDBC – Many-to-One Relationship

I can’t seem to find any reference online with regards to using a Many-To-One mapping in Spring JDBC. I just saw in the documentation that is not supported but I’m not sure if this is the case.

My example is that I want to map my AppUser to a particular Department.

For reference, AppUser joins to Department table using DEPARTMENT_ID

@Table(value="m_appuser")
public class AppUserProjectionTwo {
    @Id
    private Long id;
    private String firstname;
    private String middlename;
    private String lastname;



    @Column("DEPARTMENT_ID")
    private DepartmentProjection departmenProjection;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

However, it seems that it won’t map properly.

@Table("M_DEPARTMENT")
public class DepartmentProjection {
    @Id
    private Long id;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

The created query looks like this. I was looking for something more of the opposite in which M_APPUSER.department_ID = Department.id

[SELECT "m_appuser"."ID" AS "ID", "m_appuser"."LASTNAME" AS "LASTNAME", "m_appuser"."FIRSTNAME" AS "FIRSTNAME", "m_appuser"."MIDDLENAME" AS "MIDDLENAME", "departmenProjection"."ID" AS "DEPARTMENPROJECTION_ID" FROM "m_appuser" LEFT OUTER JOIN "M_DEPARTMENT" AS "departmenProjection" ON "departmenProjection"."DEPARTMENT_ID" = "m_appuser"."ID" WHERE "m_appuser"."FIRSTNAME" = ?];

Thanks

Answer

I just saw in the documentation that is not supported but I’m not sure if this is the case.

I can confirm it is not supported. Many-To-One relationships cross the boundaries of aggregates. References across aggregates must be modelled as ids of the referenced aggregate.

If you don’t do this Spring Data JDBC will consider the reference a One-To-One relationship and part of the same aggregate which will have effects you don’t want for a Many-To-One relationship, like the referenced entity getting deleted when the referenced entity gets deleted. Which would be correct for a One-To-One relationship within the same aggregate.

This is explained in more detail in https://spring.io/blog/2018/09/24/spring-data-jdbc-references-and-aggregates