An ordinary association to another persistent class is declared using a many-to-one element. The relational model is a many-to-one association: a foreign key in one table is referencing the primary key column(s) of the target table.

<many-to-one
        name="propertyName"
        column="column_name"
        class="ClassName"
        cascade="cascade_style"
        fetch="join|select"
        update="true|false"
        insert="true|false"
        property-ref="propertyNameFromAssociatedClass"
        access="field|property|ClassName"
        unique="true|false"
        not-null="true|false"
        optimistic-lock="true|false"
        lazy="proxy|no-proxy|false"
        not-found="ignore|exception"
        entity-name="EntityName"
        formula="arbitrary SQL expression"
        node="element-name|@attribute-name|element/@attribute|."
        embed-xml="true|false"
        index="index_name"
        unique_key="unique_key_id"
        foreign-key="foreign_key_name"
/>

name: The name of the property.

column (optional): The name of the foreign key column. This may also be specified by nested <column> element(s).

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.

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

update, insert (optional - defaults to true) specifies that the mapped columns should be included in SQL UPDATE and/or INSERT statements. Setting both to false allows a pure "derived" association whose value is initialized from some other property that maps to the same colum(s) or by a trigger or other application.

property-ref: (optional) The name of a property of the associated class that is joined to this foreign key. 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.

unique (optional): Enable the DDL generation of a unique constraint for the foreign-key column. Also, allow this to be the target of a property-ref. This makes the association multiplicity effectively one to one.

not-null (optional): Enable the DDL generation of a nullability constraint for the foreign key columns.

optimistic-lock (optional - defaults to true): Specifies that updates to this property do or do not require acquisition of the optimistic lock. In other words, dertermines if a version increment should occur when this property is dirty.

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.

not-found (optional - defaults to exception): Specifies how foreign keys that reference missing rows will be handled: ignore will treat a missing row as a null association.

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

formula (optional): an SQL expression that defines the value for a computed foreign key.

Setting a value of the cascade attribute to any meaningful value other than none will propagate certain operations to the associated object. The meaningful values are the names of Hibernate's basic operations, persist, merge, delete, save-update, evict, replicate, lock, refresh, as well as the special values delete-orphan and all and comma-separated combinations of operation names, for example, cascade="persist,merge,evict" or cascade="all,delete-orphan". See Section 10.11, “Transitive persistence” for a full explanation. Note that single valued associations (many-to-one and one-to-one associations) do not support orphan delete.

A typical many-to-one declaration looks as simple as this:

<many-to-one name="product" class="Product" column="PRODUCT_ID"/>

The property-ref attribute should only be used for mapping legacy data where a foreign key refers to a unique key of the associated table other than the primary key. This is an ugly relational model. For example, suppose the Product class had a unique serial number, that is not the primary key. (The unique attribute controls Hibernate's DDL generation with the SchemaExport tool.)

<property name="serialNumber" unique="true" type="string" column="SERIAL_NUMBER"/>

Then the mapping for OrderItem might use:

<many-to-one name="product" property-ref="serialNumber" column="PRODUCT_SERIAL_NUMBER"/>

This is certainly not encouraged, however.

If the referenced unique key comprises multiple properties of the associated entity, you should map the referenced properties inside a named <properties> element.

If the referenced unique key is the property of a component, you may specify a property path:

<many-to-one name="owner" property-ref="identity.ssn" column="OWNER_SSN"/>