How to Map Composite Keys in JPA with Hibernate Using `EmbeddedId` and `IdClass`?

Composite Keys in JPA with Hibernate

When working with relational databases, it may be necessary to map composite keys to Java classes for object-oriented programming. In the given code, the Time table has a composite key comprising levelStation and confPathID. This article will guide you on how to map this composite key using JPA and Hibernate with both EmbeddedId and IdClass annotations.

IdClass

The IdClass annotation is used to map multiple individual fields into a single composite primary key (PK). To implement this method, create a static inner class that defines the composite PK structure and implement its equals() and hashCode() methods for proper value comparison. Identify the corresponding fields in this static class with those in the entity.

public class TimePK implements Serializable {
    protected Integer levelStation;
    protected Integer confPathID;
    // ...
}

@Entity
@IdClass(TimePK.class)
public class Time {
    @Id private Integer levelStation;
    @Id private Integer confPathID;
    // ...
}

EmbeddedId

Using the EmbeddedId annotation, you can encapsulate the composite PK as an embeddable class within the entity. Within the embeddable class, define the fields constituting the composite PK and their accessors.

@Embeddable
public class TimePK {
    protected Integer levelStation;
    protected Integer confPathID;
    // ...
}

@Entity
public class Time {
    @EmbeddedId private TimePK timePK;
    // ...
}

Differences

Both IdClass and EmbeddedId annotations can be used to map composite keys, but they serve slightly different purposes:

• EmbeddedId emphasizes that the composite PK has semantic meaning as an entity within itself or is potentially reusable.
• IdClass, on the other hand, focuses on specifying that a particular combination of fields uniquely identifies an entity without implying any specific meaning.

Querying with Composite Keys

Depending on the annotation used, slightly different syntax is required when querying based on composite keys:

IdClass

select t.levelStation from Time t

EmbeddedId

select t.timePK.levelStation from Time t

References

• [JPA 1.0 Specification](https://docs.oracle.com/javaee/7/api/javax/persistence/package-summary.html)

  • Section 2.1.4 "Primary Keys and Entity Identity"
  • Section 9.1.14 "EmbeddedId Annotation"
  • Section 9.1.15 "IdClass Annotation"

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