QueryDSL / JPQL : how to build a join query? - jpa

Question

I've tried to read through the QueryDSL docs but I am still very confused. I'm accustomed to writing a lot of SQL, but this is my first real crack at using QueryDSL w/ JPQL (JPA2).
I have the following entity:
#Entity
public class Provider implements Serializable {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
#Column(name = "id")
private Long id;
#Version
#Column(name = "version")
private Integer version;
private String name;
#ManyToMany(cascade=CascadeType.ALL)
#JoinTable(name = "provider_contact", joinColumns = #JoinColumn(name = "contact_id", referencedColumnName = "id"), inverseJoinColumns = #JoinColumn(name = "provider_id", referencedColumnName = "id"))
#OrderColumn
private Collection<Contact> contact;
}
where Contact is a simple entity with an id for a pk.
#Entity
public class Contact {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
#Column(name = "id")
private Long id;
/**
* User first name
*/
#NotNull
private String firstName;
/**
* User last name
*/
#NotNull
private String lastName;
}
I'm trying to write a query which returns a Contact object given a specific Contact.id and Provider.id. If the Contact object is not a part of the Provider's Contact collection, I'm looking for a null value.
I've tried the following:
public Contact getContact( long providerId, long contactId ){
Predicate p = QProvider.provider.id.eq(providerId).and(QContact.contact.id.eq(contactId));
JPQLQuery query = new JPAQuery(em);
return query.from(QProvider.provider).innerJoin(QProvider.provider.contact).where(p).singleResult(QContact.contact);
}
but I'm getting the following error:
Caused by: java.lang.IllegalArgumentException: Undeclared path 'contact'. Add this path as a source to the query to be able to reference it.
at com.mysema.query.types.ValidatingVisitor.visit(ValidatingVisitor.java:78)
at com.mysema.query.types.ValidatingVisitor.visit(ValidatingVisitor.java:30)
at com.mysema.query.types.PathImpl.accept(PathImpl.java:94)
I'm presuming it has something to do with the fact that my predicate references QContact.contact direction and not part of the QProvider.provider.contact object, but I'm really at a loss as to figure out how this should be done.
Am I even on the right track? I'm not even sure my join is correct either.

Answer 1

This should work
public Contact getContact(long providerId, long contactId) {
QProvider provider = QProvider.provider;
QContact contact = QContact.contact;
return new JPAQuery(em).from(provider)
.innerJoin(provider.contact, contact)
.where(provider.id.eq(providerId), contact.id.eq(contactId))
.singleResult(contact);
}