Here we will discussed the all part of examples for mapping in Hibernate.
In the below example given class has a collection of child instances that for try to understand:
Example-1. Classes Parent and Child
public class Parent {
private long id;
private Set<Child> children;
// getter/setter
...
}
public class Child {
private long id;
private String name
// getter/setter
...
}
In the above example if each child has, at most, at parent the most natural mapping is a one to one mapping shows.
Example-2. One to many unidirectional Parent-Child relationship using annotations
public class Parent {
@Id
@GeneratedValue
private long id;
@OneToMany
private Set<Child> children;
// getter/setter
...
}
public class Child {
@Id
@GeneratedValue
private long id;
private String name;
// getter/setter
...
}
Example-3. One to many unidirectional Parent-Child relationship using mapping files
<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>
Example-4. Table definitions for unidirectional Parent-Child relationship
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:
Example-5. One to many bidirectional Parent-Child relationship using annotations
public class Parent {
@Id
@GeneratedValue
private long id;
@OneToMany(mappedBy="parent")
private Set<Child> children;
// getter/setter
...
}
public class Child {
@Id
@GeneratedValue
private long id;
private String name;
@ManyToOne
private Parent parent;
// getter/setter
...
}
Example-6. One to many bidirectional Parent-Child relationship using mapping files
<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>
Example-7. Table definitions for bidirectional Parent-Child relationship
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 enforce the NOT NULL constraint.
Example-8. Enforcing NOT NULL constraint in unidirectional relation using annotations
public class Parent {
@Id
@GeneratedValue
private long id;
@OneToMany(optional=false)
private Set<Child> children;
// getter/setter
...
}
public class Child {
@Id
@GeneratedValue
private long id;
private String name;
// getter/setter
...
}
Example-10. Enforcing NOT NULL constraint in unidirectional relation using mapping files
<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.
Example-11. Many to many Parent-Child relationship using annotations
public class Parent {
@Id
@GeneratedValue
private long id;
@ManyToMany
private Set<Child> children;
// getter/setter
...
}
public class Child {
@Id
@GeneratedValue
private long id;
private String name;
// getter/setter
...
}
Example-12. Many to many Parent-Child relationship using mapping files
<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:
Example-13. Table definitions for many to many releationship
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