468 words

javax.persistence.MapKey - JPA annotation

javax.persistence
Annotation MapKey

Target:
Fields (including property get methods)
Specifies the map key for associations of type java.util.Map when the map key is itself the primary key or a persistent field or property of the entity that is the value of the map.

If a persistent field or property other than the primary key is used as a map key then it is expected to have a uniqueness constraint associated with it.

The MapKeyClass annotation is not used when MapKey is specified and vice versa.

Example 1:

    @Entity
    public class Department {
        ...
        @OneToMany(mappedBy="department")
        @MapKey  // map key is primary key
        public Map<Integer, Employee> getEmployees() {... }
        ...
    }
 
    @Entity
    public class Employee {
        ...
        @Id Integer getEmpId() { ... }
        @ManyToOne
        @JoinColumn(name="dept_id")
        public Department getDepartment() { ... }
        ...
    }

Example 2:

    @Entity
        public class Department {
        ...
        @OneToMany(mappedBy="department")
        @MapKey(name="name")
        public Map<String, Employee> getEmployees() {... }
        ...
    }
 
    @Entity
        public class Employee {
        @Id public Integer getEmpId() { ... }
        ...
        @ManyToOne
        @JoinColumn(name="dept_id")
        public Department getDepartment() { ... }
        ...
    }

Since:
JPA 1.0
String name
(Optional) The name of the persistent field or property of the associated entity that is used as the map key.
(Optional) The name of the persistent field or property of the associated entity that is used as the map key.

Default: If the name element is not specified, the primary key of the associated entity is used as the map key. If the primary key is a composite primary key and is mapped as IdClass, an instance of the primary key class is used as the key.

Default value:
""
Since:
JPA 1.0

This documentation page is derived (with some adjustments) from the open source JPA 2 RI (EclipseLink)
and is available under the terms of the Eclipse Public License, v. 1.0 and Eclipse Distribution License, v. 1.0.
Object Relational Mapping (ORM) JPA 2 providers include Hibernate, EclipseLink, TopLink, OpenJPA and DataNucleus.
Object DB is not an ORM JPA implementation but an Object Database for Java with built in JPA 2 support.