Fields That Are Objects

We are going to use the Customer and Address classes in our examples for this chapter. Up to this point we have a number of fields configured in our entities, including a few relationships.

@Entity
public class Customer {
    @Id
    @GeneratedValue(strategy= GenerationType.IDENTITY)
    private int id;

    @Column(name="first_name")
    private String firstName;

    @Column(name="last_name")
    private String lastName;

    private String email;

    @Column(name="create_date")
    @Temporal(TemporalType.DATE)
    private Date createDate;

    @OneToOne
    @JoinColumn(name="address_id")
    private Address address;

  // ...
}

@Entity
public class Address {
    @Id
    @GeneratedValue(strategy= GenerationType.IDENTITY)
    private int id;

    @Column(name="address")
    private String street;

    @Column(name="address2")
    private String street2;

    @Column(name="state_province")
    private String state;

    private String city;

    @Column(name="postal_code")
    private String postalCode;

    private String phone;

    @ManyToOne
    @JoinColumn(name="country_code")
    private Country country;

  // ...
}

WHERE Clause with Nested Objects

We know that if we wanted to write a query to select the Customers with the first name "Leslie", we could utilize the .firstName property.

String query = "SELECT c FROM Customer c where c.firstName= 'Leslie'";

Nested objects are no different. We can use . notation to dig as deep into an object as we desire. For example, if we wanted to select all the Customers that have a state of Illinois we could use the path .address.state to access that value.

String query = "SELECT c FROM Customer c where c.address.state= 'Illinois'";

Objects in the SELECT List

We know that if we wanted to write a query to select the firstName of the Customer with id 5 we could:

String query = "SELECT c.firstName FROM Customer c where c.id= 5";
List<String> results =
  em.createQuery(query, String.class)
    .getResultList();

• The return value of this query is a List<String> due to the fact that we are not returning the entire Customer object, just one of its properties that happens to be a String.

What if we wanted to query for the address of Customer with id 5?

String query = "SELECT c.address FROM Customer c where c.id=5";
List<Address> results =
  em.createQuery(query, Address.class)
    .getResultList();

* Notice: The return value of this query is a List<Address> due to the fact that we are not returning the entire Customer object. Instead we are returning one of its properties that happens to be an Address object. The type of the collection that is returned from a query is determined by the property in the select list.

We could continue drilling down and return just the state of the address for Customer with id 5:

String query = "SELECT c.address.state FROM Customer c where c.id=5";
List<String> results =
  em.createQuery(query, String.class)
    .getResultList();

* Notice: We are chaining through the Customer -> Address -> state to grab just the specific property that we want to return from this query. Due to the fact that we are returning c.address.state, the returned collection would be typed List<String>.

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