Hibernate Unidirectional associations

7.2. Unidirectional associations

7.2.1. many to one

A unidirectional many-to-one association is the most common kind of unidirectional association.

<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<many-to-one name="address"
column="person_address_id"
not-null="true"/>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
</class>

create table Person ( 

    personId bigint not null primary key, 

    person_address_id bigint not null,

    CONSTRAINT fk_person_address FOREIGN KEY (persion_address_id) REFERENCES address (addressId)
)

create table Address ( addressId bigint not null primary key )

7.2.2. one to one

A unidirectional one-to-one association on a foreign key is almost identical. The only difference is the column unique constraint.

<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<many-to-one name="address"
column="addressId"
unique="true"
not-null="true"/>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
</class>

create table Person ( personId bigint not null primary key, addressId bigint not null unique )
create table Address ( addressId bigint not null primary key )

A unidirectional one-to-one association on a primary key usually uses a special id generator. (Notice that we've reversed the direction of the association in this example.)

<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
</class>
<class name="Address">
<id name="id" column="personId">
<generator class="foreign">
<param name="property">person</param>
</generator>
</id>
<one-to-one name="person" constrained="true"/>
</class>

create table Person ( personId bigint not null primary key )
create table Address ( personId bigint not null primary key )

7.2.3. one to many

A unidirectional one-to-many association on a foreign key is a very unusual case, and is not really recommended.

<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<set name="addresses">
<key column="address_person_id"
not-null="true"/>
<one-to-many class="Address"/>
</set>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
</class>

create table Person ( personId bigint not null primary key )

create table Address ( addressId bigint not null primary key, address_person_id bigint not null )

alter table Address address_persion_id ADD CONSTRAINT fk_address_person FOREIGN KEY(address_person_id)
REFERENCES person(personId);

We think it's better to use a join table for this kind of association.