JPA NamedQuery to obtain a referenced object without executing join

Question

Say I have three entities: Customer, Product and Order. Each Order has a ManyToOne association to a Customer and a ManyToOne association to a Product. Associations are unidirectional from Order to Customer and to Product respectively. So the database looks like:

Customer
+----+-----+
| ID | ... |
+----+-----+

Product
+----+-----+
| ID | ... |
+----+-----+

Order
+----+-------------+------------+-----+
| ID | CUSTOMER_ID | PRODUCT_ID | ... |
+----+-------------+------------+-----+

Now I want to write a NamedQuery to retrieve all the Customer who ordered a particular Product. Can I refer the Customer object in the query without executing a join between Customer and Order? Can I write something like:

@NamedQuery(
name =  "GetCustomersByProduct",
query = "SELECT o.customer"
        "FROM Order o "+
        "WHERE  o.productId = :productId"
)

Or is it necessary to execute a join? What's the best way to do this?

Answer 1

Your query is right and only thing you need to get distinct customer since same product customer may have ordered more than once

query = "SELECT distinct(o.customer)"
    "FROM Order o "+
    "WHERE  o.productId = :productId"

Get Customer :

    Query qry = em.createNamedQuery("GetCustomersByProduct");
    List<Customer> resultList = qry.getResultList();

Answer 2

Good practice to avoid JOIN is usage of exists (see oracle doc: http://docs.oracle.com/html/E24396_01/ejb3_langref.html#ejb3_langref_exists ), most databases optimize EXISTS in compare with JOIN.