JPA relationships in multi-module architecture (MSA)

JPA in normal cases

Well in normal cases, we add JPA relationships like @ManyToOne, @OnetoMany, etc to our primary identifiers.

For example in a unidirectional @ManyToOne relationship between Project and Member in my toy project:

public class Project extends BaseTimeEntity {

    @Id
    @GeneratedValue(strategy = IDENTITY)
    @Column(name = "project_id")
    private Long id;

    @ManyToOne
    @JoinColumn(name = "member_id")
    private Long member_id;
}

And we have the relevant Member entity in the same module.

JPA in multi-module

Now, in multi-module architecture, we actually remove this @ManyToOne annotation because we don’t have the Member entity in the same module that contains Project entity.

So we adjust it like this:

public class Project extends BaseTimeEntity {

    @Id
    @GeneratedValue(strategy = IDENTITY)
    @Column(name = "project_id")
    private Long id;
    
    @JoinColumn(name = "member_id")
    private Long member_id;
}