EntityManager query by joinColumn

Question

I have a Login entity and a Customer entity. Login.username is a foreign key in the customer table. Hence the following line in the Java Customer POJO

@OneToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "username", nullable = false)
private Login login;

My question is this: Is there a simple way to query the customer table using username? Or must I first fetch login by username and then customer by login?

Here is the JPA criteria query. And, yes, I would prefer to use criteria query.

public Customer getCustomerByUsername(String username) throws EntityNotFoundException {
    CriteriaBuilder criteriaBuilder = entityManager.getCriteriaBuilder();
    CriteriaQuery<Customer> criteriaQuery = criteriaBuilder.createQuery(Customer.class);
    Root<Customer> root = criteriaQuery.from(Customer.class);
    Path<String> path = root.<String>get("username");
    criteriaQuery.where(criteriaBuilder.equal(path, username));
    return entityManager.createQuery(criteriaQuery).getSingleResult();
}

The line Path<String> path = root.<String>get("username") is throwing an exception saying that username ... is not present.

Answer 1

The correct solution with JPQL is

Query q = entityManager.createQuery("SELECT c FROM Customer c WHERE c.login.username = :username");
    q.setParameter("username", username);
    return (Customer) q.getSingleResult();

Answer 2

Query q = entityManager.createQuery("SELECT c FROM Customer c JOIN Login l ON c.login=l WHERE l.username = :username");
q.setParameter("username",username);
List<Customer> customerList = q.getResultList();

There are a few tricks to keep in mind: this all is about the objects, and not the underlying DB. So the names represent classes, instances, and fields, and they are all CaseSensitive... The error you got meant that your Customer class didn't have a userName field - which is true, since Login has that... Sadly, I can't test it, but the logic and intentions should be clear.