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

Mapping Composite Keys with JPA and Hibernate

In the provided SQL schema, the Time table has a composite primary key consisting of levelStation and confPathID fields. Mapping this type of key in JPA and Hibernate requires special annotations to represent the compound primary key.

Using the IdClass Annotation

One option is to define a Java class for the composite key using the @IdClass annotation. The class should have fields corresponding to the primary key columns, as well as getters, setters, equals, and hashCode methods:

public class TimePK implements Serializable {
    private Integer levelStation;
    private Integer confPathID;

    // Getters, setters, equals, hashCode
}

The entity class is then annotated with @IdClass and the composite key object:

@Entity
@IdClass(TimePK.class)
class Time implements Serializable {
    @Id
    private Integer levelStation;
    @Id
    private Integer confPathID;

    // Other properties
}

Using the EmbeddedId Annotation

Alternatively, you can use the @EmbeddedId annotation to define a class containing the composite key:

@Embeddable
public class TimePK implements Serializable {
    private Integer levelStation;
    private Integer confPathID;

    // Getters, setters, equals, hashCode
}

The entity class is then annotated with @EmbeddedId and the embedded composite key object:

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

    // Other properties
}

Differences between IdClass and EmbeddedId

Both @IdClass and @EmbeddedId allow mapping of composite primary keys. However, there are some minor differences:

• @IdClass specifies that a unique combination of fields forms the primary key, but they do not have a distinct meaning.
• @EmbeddedId suggests that the composite key is a meaningful entity in its own right.

The choice between @IdClass and @EmbeddedId depends on the specific semantics of your application.

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