A unidirectional one-to-many association on a join table
is much preferred. Notice that by specifying unique="true"
,
we have changed the multiplicity from many-to-many to one-to-many.
<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <set name="addresses" table="PersonAddress"> <key column="personId"/> <many-to-many column="addressId" unique="true" 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 PersonAddress ( personId not null, addressId bigint not null primary key ) create table Address ( addressId bigint not null primary key )
A unidirectional many-to-one association on a join table is quite common when the association is optional.
<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <join table="PersonAddress" optional="true"> <key column="personId" unique="true"/> <many-to-one name="address" column="addressId" not-null="true"/> </join> </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 PersonAddress ( personId bigint not null primary key, addressId bigint not null ) create table Address ( addressId bigint not null primary key )
A unidirectional one-to-one association on a join table is extremely unusual, but possible.
<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <join table="PersonAddress" optional="true"> <key column="personId" unique="true"/> <many-to-one name="address" column="addressId" not-null="true" unique="true"/> </join> </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 PersonAddress ( personId bigint not null primary key, addressId bigint not null unique ) create table Address ( addressId bigint not null primary key )
Finally, we have a unidirectional many-to-many association.
<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <set name="addresses" table="PersonAddress"> <key column="personId"/> <many-to-many column="addressId" 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 PersonAddress ( personId bigint not null, addressId bigint not null, primary key (personId, addressId) ) create table Address ( addressId bigint not null primary key )