One to One

layout: default title: @OneToOne

Use the @OneToOne annotation to describe a one-to-one relationship.

Annotate the defining field in the owning entity to specify mapping details.
The owning entity is the one whose corresponding database table contains the foreign key which models the relationship.
Use the @JoinColumn annotation to specify the foreign key column name.

Customer SQL Table

@Entity
public class Customer {
     // ...
     @OneToOne
     @JoinColumn(name="address_id")
     private Address address;
     // ...
}

In the non-owning entity of a bi-directional relationship, add the @OneToOne annotation above the field that references the owning entity.
Use the mappedBy element on the @OneToOne annotation to specify the defining field in the owning entity.

@Entity
public class Address {
     // ...
     @OneToOne(mappedBy="address")
     private Customer customer;
     // ...
}

Digging Deeper¶

One To One

For a uni-directional relationship, omit the second @OneToOne annotation (with the mappedBy element), and the field it decorates.  You only need a field and mapping in the owning entity of a uni-directional relationship.

Drill¶

Delete the field and the gets/sets for addressId from your Customer class.
```
@Column(name="address_id")
public int addressId;
```
This will break the CustomerRelationshipClient due to the fact that the gets/sets we are calling no longer exist. Because this was just a demo, it's not a problem. Comment out the main to get rid of the errors.
Create a uni-directional, one-to-one relationship between your Customer and Address entities.

First, in Customer add a field of type Address annotated with @OneToOne and @JoinColumn.

Here is the customer table for you to reference:

+-------------+----------------------+------+-----+-------------------+-----------------------------+
| Field       | Type                 | Null | Key | Default           | Extra                       |
+-------------+----------------------+------+-----+-------------------+-----------------------------+
| id          | int(10) unsigned     | NO   | PRI | NULL              | auto_increment              |
| store_id    | smallint(5) unsigned | NO   | MUL | NULL              |                             |
| first_name  | varchar(45)          | NO   |     | NULL              |                             |
| last_name   | varchar(45)          | NO   | MUL | NULL              |                             |
| email       | varchar(50)          | YES  |     | NULL              |                             |
| address_id  | int(10) unsigned     | NO   | MUL | NULL              |                             |
| active      | tinyint(1)           | NO   |     | 1                 |                             |
| create_date | datetime             | NO   |     | NULL              |                             |
| last_update | timestamp            | NO   |     | CURRENT_TIMESTAMP | on update CURRENT_TIMESTAMP |
+-------------+----------------------+------+-----+-------------------+-----------------------------+

Also, replace the get/set methods with new versions.
Because this is a uni-directional relationship, the Address entity remains unchanged.
Update CustomerTest so that it tests your new relationship.

  @Test
  public void test_customer_to_address_association() {
     Customer cust = em.find(Customer.class, 2);
     Address address = cust.getAddress();
     assertEquals("1121 Loja Avenue", address.getStreet());
     assertEquals("", address.getStreet2());
     assertEquals("San Bernardino", address.getCity());
     assertEquals("17886", address.getPostalCode());
  }

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