A one-to-one association to another persistent class is declared using a one-to-one element.

<one-to-one
        name="propertyName"
        class="ClassName"
        cascade="cascade_style"
        constrained="true|false"
        fetch="join|select"
        property-ref="propertyNameFromAssociatedClass"
        access="field|property|ClassName"
        formula="any SQL expression"
        lazy="proxy|no-proxy|false"
        entity-name="EntityName"
        node="element-name|@attribute-name|element/@attribute|."
        embed-xml="true|false"
        foreign-key="foreign_key_name"
/>

name: The name of the property.

class (optional - defaults to the property type determined by reflection): The name of the associated class.

cascade (optional) specifies which operations should be cascaded from the parent object to the associated object.

constrained (optional) specifies that a foreign key constraint on the primary key of the mapped table references the table of the associated class. This option affects the order in which save() and delete() are cascaded, and determines whether the association may be proxied (it is also used by the schema export tool).

fetch (optional - defaults to select): Chooses between outer-join fetching or sequential select fetching.

property-ref: (optional) The name of a property of the associated class that is joined to the primary key of this class. If not specified, the primary key of the associated class is used.

access (optional - defaults to property): The strategy Hibernate should use for accessing the property value.

formula (optional): Almost all one to one associations map to the primary key of the owning entity. In the rare case that this is not the case, you may specify a some other column, columns or expression to join on using an SQL formula. (See org.hibernate.test.onetooneformula for an example.)

lazy (optional - defaults to proxy): By default, single point associations are proxied. lazy="no-proxy" specifies that the property should be fetched lazily when the instance variable is first accessed (requires build-time bytecode instrumentation). lazy="false" specifies that the association will always be eagerly fetched. Note that if constrained="false", proxying is impossible and Hibernate will eager fetch the association!

entity-name (optional): The entity name of the associated class.

There are two varieties of one-to-one association:

Primary key associations don't need an extra table column; if two rows are related by the association then the two table rows share the same primary key value. So if you want two objects to be related by a primary key association, you must make sure that they are assigned the same identifier value!

For a primary key association, add the following mappings to Employee and Person, respectively.

<one-to-one name="person" class="Person"/>

<one-to-one name="employee" class="Employee" constrained="true"/>

Now we must ensure that the primary keys of related rows in the PERSON and EMPLOYEE tables are equal. We use a special Hibernate identifier generation strategy called foreign:

<class name="person" table="PERSON">
    <id name="id" column="PERSON_ID">
        <generator class="foreign">
            <param name="property">employee</param>
        </generator>
    </id>
    ...
    <one-to-one name="employee"
        class="Employee"
        constrained="true"/>
</class>

A newly saved instance of Person is then assigned the same primary key value as the Employee instance refered with the employee property of that Person.

Alternatively, a foreign key with a unique constraint, from Employee to Person, may be expressed as:

<many-to-one name="person" class="Person" column="PERSON_ID" unique="true"/>

And this association may be made bidirectional by adding the following to the Person mapping:

<one-to-one name"employee" class="Employee" property-ref="person"/>