How to Map Composite Keys in JPA and Hibernate using @EmbeddedId vs. @IdClass?

Mapping Composite Keys with JPA and Hibernate

In the provided code, a composite key is defined for the Time table with levelStation and confPathID as its components. To map this composite key in JPA and Hibernate, you have two options: @EmbeddedId and @IdClass.

Using @IdClass

The @IdClass annotation allows you to map multiple fields of the entity to form the composite primary key. This requires defining a class that represents the primary key. In this case, the TimePK class can be defined as follows:

@IdClass(TimePK.class)
public class Time implements Serializable {
    // ...
}

Here, TimePK is a separate class that must define a no-arg constructor, getters/setters for the primary key fields, and equals() and hashCode() methods.

Using @EmbeddedId

The @EmbeddedId annotation maps an embedded class to the primary key of the entity. As opposed to @IdClass, this approach combines the entity class and the primary key class. Here's how you would use @EmbeddedId:

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

In this case, TimePK is an inner class of the Time entity and must also define a no-arg constructor, getters/setters for the key fields, and equals() and hashCode() methods.

Differences Between @IdClass and @EmbeddedId

The main difference between @IdClass and @EmbeddedId is their impact on query syntax. With @IdClass, you would access the primary key field directly via the entity, while with @EmbeddedId, you would access it through a getter on the embedded class.

When choosing between the two, @EmbeddedId conveys the fact that the primary key has a meaningful identity in your domain model, while @IdClass suggests that the primary key is simply a combination of fields that uniquely identifies the entity.

References

• JPA 1.0 Specification:

  • 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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