How to correctly do a manytomany join table in JPA? - jpa

Question

I need 3 entities: User, Contract (which are a many to many relation) and a middle entity: UserContract (this is needed to store some fields).
What I want to know is the correct way to define the relationships between these entities in JPA/EJB 3.0 so that the operations (persist, delete, etc) are OK.
For example, I want to create a User and its contracts and persist them in a easy way.
Currently what I have is this:
In User.java:
#OneToMany(mappedBy = "user", fetch = FetchType.LAZY)
private List<UserContract> userContract;
In Contract.java:
#OneToMany(mappedBy = "contract", fetch = FetchType.LAZY)
private Collection<UserContract> userContract;
And my UserContract.java:
#Entity
public class UserContract {
#EmbeddedId
private UserContractPK userContractPK;
#ManyToOne(optional = false)
private User user;
#ManyToOne(optional = false)
private Contract contract;
And my UserContractPK:
#Embeddable
public class UserContractPK implements Serializable {
#Column(nullable = false)
private long idContract;
#Column(nullable = false)
private String email;
Is this the best way to achieve my goals?

Answer 1

Everything looks right. My advice is to use #MappedSuperclass on top of #EmbeddedId:
#MappedSuperclass
public abstract class ModelBaseRelationship implements Serializable {
#Embeddable
public static class Id implements Serializable {
public Long entityId1;
public Long entityId2;
#Column(name = "ENTITY1_ID")
public Long getEntityId1() {
return entityId1;
}
#Column(name = "ENTITY2_ID")
public Long getEntityId2() {
return entityId2;
}
public Id() {
}
public Id(Long entityId1, Long entityId2) {
this.entityId1 = entityId1;
this.entityId2 = entityId2;
}
}
protected Id id = new Id();
#EmbeddedId
public Id getId() {
return id;
}
protected void setId(Id theId) {
id = theId;
}
}
I omitted obvious constructors/setters for readability. Then you can define UserContract as
#Entity
#AttributeOverrides( {
#AttributeOverride(name = "entityId1", column = #Column(name = "user_id")),
#AttributeOverride(name = "entityId2", column = #Column(name = "contract_id"))
})
public class UserContract extends ModelBaseRelationship {
That way you can share primary key implementation for other many-to-many join entities like UserContract.