Composite Identifiers
Hibernate also allows us to define composite identifiers. FYI, a composite id is represented by a primary key class with one or more persistent attributes.
Here are the conditions which a primary class need to fulfil:
In the above example the EmployeeEntry object has an EmployeeEntryPK primary id formed of two attributes: empId and deptId.
@IdClass
The @IdClass annotation is similar to the @EmbeddedId, except the attributes are defined in the main entity class using @Id for each one. Let's rewrite the above example:
In the above example, both types of composite ids, the primary key class can also contain @ManyToOne attributes.
Hibernate also allows defining primary-keys made up of @ManyToOne associations combined with @Id annotation. In this case, the entity class should also fulfill the conditions of a primary-key class. The disadvantage of this method is that there's no separation between the entity object and the identifier.
Derived Identifiers
They are obtained from an entity's association using the @MapsId annotation.
-K Himaanshu Shuklaa..Hibernate also allows us to define composite identifiers. FYI, a composite id is represented by a primary key class with one or more persistent attributes.
Here are the conditions which a primary class need to fulfil:
- It should be defined using @EmbeddedId or @IdClass annotations
- It should be public, serializable and have a public no-arg constructor
- It should implement equals() and hashCode() methods
- The class's attributes can be basic, composite or ManyToOne while avoiding collections and OneToOne attributes.
In the above example the EmployeeEntry object has an EmployeeEntryPK primary id formed of two attributes: empId and deptId.
@IdClass
The @IdClass annotation is similar to the @EmbeddedId, except the attributes are defined in the main entity class using @Id for each one. Let's rewrite the above example:
In the above example, both types of composite ids, the primary key class can also contain @ManyToOne attributes.
Hibernate also allows defining primary-keys made up of @ManyToOne associations combined with @Id annotation. In this case, the entity class should also fulfill the conditions of a primary-key class. The disadvantage of this method is that there's no separation between the entity object and the identifier.
Derived Identifiers
They are obtained from an entity's association using the @MapsId annotation.
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