How to use JPA Criteria API in JOIN

Question

• I have five tables: Company, Product/Service, Address, Country and City.
• A Company can have n products with category, 1 address with 1 country and 1 city inside the address entity.
• A user has chosen "England - Leeds".
• I know now that I have to select every companies from db where city is Leeds and populate product/service-list with those companies' products or services. After that user can select for instance dentist from the third list.
• After that I know Enlgand - Leeds - Dentist and I have to populate the last list with compenies (dentists in Leeds)

---

public class Company implements java.io.Serializable {

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Short companyId;
@OneToOne(cascade=CascadeType.PERSIST)
private Address address;
private String companyName;
@OneToMany(fetch = FetchType.LAZY, mappedBy = "company",cascade=CascadeType.PERSIST)
private Set<Product> products = new HashSet<Product>(0);

---

public class Product implements java.io.Serializable {

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
@Column(name = "productId", unique = true, nullable = false)
private Integer productId;
private Short branchId;
private String productName;
private String sku;
private String category; ------> I am using this field in company search (dentists, garages etc.)

How can I query only those companies which have products with category dentist?

CriteriaBuilder criteriaBuilder = em.getCriteriaBuilder();

CriteriaQuery<Company> criteria = criteriaBuilder.createQuery( Company.class );
Root<Company> companyRoot = criteria.from( Company.class );
//criteria.select(companyRoot);

TypedQuery<Company> q = em.createQuery(criteria);
List<Company> results = q.getResultList();

Now I've got every company, how can I select only the companies with the correct category? I think I will need JOIN but how I don't know how to use it.

Answer 1

use join(),

criteriaBuilder.equal(companyRoot.join("products").get("category"), "dentist")

See, http://en.wikibooks.org/wiki/Java_Persistence/Querying#Joining.2C_querying_on_a_OneToMany_relationship