2.7.3. @OneToOne
<div class="paragraph">
The `@OneToOne` association can either be unidirectional or bidirectional.
A unidirectional association follows the relational database foreign key semantics, the client-side owning the relationship.
A bidirectional association features a `mappedBy` `@OneToOne` parent side too.
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##### Unidirectional `@OneToOne`
<div id="associations-one-to-one-unidirectional-example" class="exampleblock">
<div class="title">Example 131. Unidirectional `@OneToOne`</div>
<div class="content">
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<div class="content">
<pre class="prettyprint highlight">`@Entity(name = "Phone")
public static class Phone {
@Id
@GeneratedValue
private Long id;
@Column(name = "`number`")
private String number;
@OneToOne
@JoinColumn(name = "details_id")
private PhoneDetails details;
public Phone() {
}
public Phone(String number) {
this.number = number;
}
public Long getId() {
return id;
}
public String getNumber() {
return number;
}
public PhoneDetails getDetails() {
return details;
}
public void setDetails(PhoneDetails details) {
this.details = details;
}
}
@Entity(name = "PhoneDetails")
public static class PhoneDetails {
@Id
@GeneratedValue
private Long id;
private String provider;
private String technology;
public PhoneDetails() {
}
public PhoneDetails(String provider, String technology) {
this.provider = provider;
this.technology = technology;
}
public String getProvider() {
return provider;
}
public String getTechnology() {
return technology;
}
public void setTechnology(String technology) {
this.technology = technology;
}
}`</pre>
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<div class="content">
<pre class="prettyprint highlight">`CREATE TABLE Phone (
id BIGINT NOT NULL ,
number VARCHAR(255) ,
details_id BIGINT ,
PRIMARY KEY ( id )
)
CREATE TABLE PhoneDetails (
id BIGINT NOT NULL ,
provider VARCHAR(255) ,
technology VARCHAR(255) ,
PRIMARY KEY ( id )
)
ALTER TABLE Phone
ADD CONSTRAINT FKnoj7cj83ppfqbnvqqa5kolub7
FOREIGN KEY (details_id) REFERENCES PhoneDetails`</pre>
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<div class="paragraph">
From a relational database point of view, the underlying schema is identical to the unidirectional `@ManyToOne` association,
as the client-side controls the relationship based on the foreign key column.
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<div class="paragraph">
But then, it’s unusual to consider the `Phone` as a client-side and the `PhoneDetails` as the parent-side because the details cannot exist without an actual phone.
A much more natural mapping would be if the `Phone` were the parent-side, therefore pushing the foreign key into the `PhoneDetails` table.
This mapping requires a bidirectional `@OneToOne` association as you can see in the following example:
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<div class="sect4">
##### Bidirectional `@OneToOne`
<div id="associations-one-to-one-bidirectional-example" class="exampleblock">
<div class="title">Example 132. Bidirectional `@OneToOne`</div>
<div class="content">
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight">`@Entity(name = "Phone")
public static class Phone {
@Id
@GeneratedValue
private Long id;
@Column(name = "`number`")
private String number;
@OneToOne(mappedBy = "phone", cascade = CascadeType.ALL, orphanRemoval = true, fetch = FetchType.LAZY)
private PhoneDetails details;
public Phone() {
}
public Phone(String number) {
this.number = number;
}
public Long getId() {
return id;
}
public String getNumber() {
return number;
}
public PhoneDetails getDetails() {
return details;
}
public void addDetails(PhoneDetails details) {
details.setPhone( this );
this.details = details;
}
public void removeDetails() {
if ( details != null ) {
details.setPhone( null );
this.details = null;
}
}
}
@Entity(name = "PhoneDetails")
public static class PhoneDetails {
@Id
@GeneratedValue
private Long id;
private String provider;
private String technology;
@OneToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "phone_id")
private Phone phone;
public PhoneDetails() {
}
public PhoneDetails(String provider, String technology) {
this.provider = provider;
this.technology = technology;
}
public String getProvider() {
return provider;
}
public String getTechnology() {
return technology;
}
public void setTechnology(String technology) {
this.technology = technology;
}
public Phone getPhone() {
return phone;
}
public void setPhone(Phone phone) {
this.phone = phone;
}
}`</pre>
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<div class="content">
<pre class="prettyprint highlight">`CREATE TABLE Phone (
id BIGINT NOT NULL ,
number VARCHAR(255) ,
PRIMARY KEY ( id )
)
CREATE TABLE PhoneDetails (
id BIGINT NOT NULL ,
provider VARCHAR(255) ,
technology VARCHAR(255) ,
phone_id BIGINT ,
PRIMARY KEY ( id )
)
ALTER TABLE PhoneDetails
ADD CONSTRAINT FKeotuev8ja8v0sdh29dynqj05p
FOREIGN KEY (phone_id) REFERENCES Phone`</pre>
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<div class="paragraph">
This time, the `PhoneDetails` owns the association, and, like any bidirectional association, the parent-side can propagate its lifecycle to the child-side through cascading.
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<div id="associations-one-to-one-bidirectional-lifecycle-example" class="exampleblock">
<div class="title">Example 133. Bidirectional `@OneToOne` lifecycle</div>
<div class="content">
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight">`Phone phone = new Phone( "123-456-7890" );
PhoneDetails details = new PhoneDetails( "T-Mobile", "GSM" );
phone.addDetails( details );
entityManager.persist( phone );`</pre>
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<div class="content">
<pre class="prettyprint highlight">`INSERT INTO Phone ( number, id )
VALUES ( '123 - 456 - 7890', 1 )
INSERT INTO PhoneDetails ( phone_id, provider, technology, id )
VALUES ( 1, 'T - Mobile, GSM', 2 )`</pre>
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<div class="paragraph">
When using a bidirectional `@OneToOne` association, Hibernate enforces the unique constraint upon fetching the child-side.
If there are more than one children associated with the same parent, Hibernate will throw a constraint violation exception.
Continuing the previous example, when adding another `PhoneDetails`, Hibernate validates the uniqueness constraint when reloading the `Phone` object.
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<div id="associations-one-to-one-bidirectional-constraint-example" class="exampleblock">
<div class="title">Example 134. Bidirectional `@OneToOne` unique constraint</div>
<div class="content">
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight">`PhoneDetails otherDetails = new PhoneDetails( "T-Mobile", "CDMA" );
otherDetails.setPhone( phone );
entityManager.persist( otherDetails );
entityManager.flush();
entityManager.clear();
//throws javax.persistence.PersistenceException: org.hibernate.HibernateException: More than one row with the given identifier was found: 1
phone = entityManager.find( Phone.class, phone.getId() );`</pre>
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