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7.4. Collection examples
This section covers collection examples.
The following class has a collection of
Child
instances:
package eg; import java.util.Set; public class Parent { private long id; private Set children; public long getId() { return id; } private void setId(long id) { this.id=id; } private Set getChildren() { return children; } private void setChildren(Set children) { this.children=children; } .... .... }
If each child has, at most, one parent, the most natural mapping is a one-to-many association:
<hibernate-mapping> <class name="Parent"> <id name="id"> <generator class="sequence"/> </id> <set name="children"> <key column="parent_id"/> <one-to-many class="Child"/> </set> </class> <class name="Child"> <id name="id"> <generator class="sequence"/> </id> <property name="name"/> </class> </hibernate-mapping>
This maps to the following table definitions:
create table parent ( id bigint not null primary key ) create table child ( id bigint not null primary key, name varchar(255), parent_id bigint ) alter table child add constraint childfk0 (parent_id) references parent
If the parent is required, use a bidirectional one-to-many association:
<hibernate-mapping> <class name="Parent"> <id name="id"> <generator class="sequence"/> </id> <set name="children" inverse="true"> <key column="parent_id"/> <one-to-many class="Child"/> </set> </class> <class name="Child"> <id name="id"> <generator class="sequence"/> </id> <property name="name"/> <many-to-one name="parent" class="Parent" column="parent_id" not-null="true"/> </class> </hibernate-mapping>
Notice the
NOT NULL
constraint:
create table parent ( id bigint not null primary key ) create table child ( id bigint not null primary key, name varchar(255), parent_id bigint not null ) alter table child add constraint childfk0 (parent_id) references parent
Alternatively, if this association must be unidirectional you can declare the
NOT NULL
constraint on the <key>
mapping:
<hibernate-mapping> <class name="Parent"> <id name="id"> <generator class="sequence"/> </id> <set name="children"> <key column="parent_id" not-null="true"/> <one-to-many class="Child"/> </set> </class> <class name="Child"> <id name="id"> <generator class="sequence"/> </id> <property name="name"/> </class> </hibernate-mapping>
On the other hand, if a child has multiple parents, a many-to-many association is appropriate:
<hibernate-mapping> <class name="Parent"> <id name="id"> <generator class="sequence"/> </id> <set name="children" table="childset"> <key column="parent_id"/> <many-to-many class="Child" column="child_id"/> </set> </class> <class name="Child"> <id name="id"> <generator class="sequence"/> </id> <property name="name"/> </class> </hibernate-mapping>
Table definitions:
create table parent ( id bigint not null primary key ) create table child ( id bigint not null primary key, name varchar(255) ) create table childset ( parent_id bigint not null, child_id bigint not null, primary key ( parent_id, child_id ) ) alter table childset add constraint childsetfk0 (parent_id) references parent alter table childset add constraint childsetfk1 (child_id) references child
For more examples and a complete explanation of a parent/child relationship mapping, see Chapter 22, Example: Parent/Child for more information.
Even more complex association mappings are covered in the next chapter.