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() { ... }
       ...
   }