How do I map this relationship using JPA/Hibernate?

Question

I have two tables that share a common key. What I'd like is that when I load the class for the first table, I would also get a list of classes corresponding to the second table where they share the key.

More specifically: tableA has an id (id1) that exists in multiple entries of tableB. However, tableB uses a composite key and uses both id1 and id2 as keys.

What I'd like to achieve is that when the POJO for tableA is loaded, i also get all entries of table B where id1 from table A equals id1 in table B. Something like:

public class TableA {
  private String id1;
  private List<TableB> list; // where id1 in tableA == id1 in tableB
}


table A id column: 
id1

table B composite id: 
id1 // same as for tableA
id2

Alan Hay Alan Hay · Accepted Answer · 2018-11-24T10:26:03

Class B will have a composite Identifier partially dependent on A. This can be mapped using an EmbeddedID or by specifiyng an ID class. Using an ID class would look like the below:

Entity A

@Entity
public class A {
    @Id
    @Column(name = "id1")
    private Long id1;

    @OneToMany(mappedBy = "a")
    private Set<B> bs;
}

Entity B

@Entity
@IdClass(Bid.class)
public class B {
    @Id
    @ManyToOne
    @JoinColumn(name = "id1")
    private A a;

    @Id
    @Column(name = "id1")
    private Long id2;
}

ID Class for B

public class Bid implements Serializable{
    //must be of same type as id of target entity A
    private Long a;
    private Long id2;

    // **must** implement equals and hash code
}

How do I map this relationship using JPA/Hibernate?

1 Answers