How to Implement Composite Primary Keys for Versioned Data Entries in JPA?

Creating and Handling Composite Primary Keys in JPA

Scenario:

Consider a scenario where you need to store multiple versions of the same data entry, represented by id and Version columns. The goal is to create an entity that can duplicate entries with different versions.

Entity Definition:

To define an entity with a composite primary key, you can use the following approach:

1. Embedded Class: Create an embedded class containing the key fields. This class will be used as an EmbeddedId in the entity.

<code class="java">@Embeddable
public class MyKey implements Serializable {

    @Column(name = "Id", nullable = false)
    private int id;

    @Column(name = "Version", nullable = false)
    private int version;

    // Getters and setters
}</code>

2. Entity Class: The entity class will have a reference to the embedded class as its primary key.

<code class="java">@Entity
public class YourEntity {

    @EmbeddedId
    private MyKey myKey;

    @Column(name = "ColumnA")
    private String columnA;

    // Getters and setters
}</code>

Another Approach:

Alternatively, the @IdClass annotation can be used to define a composite primary key. This approach allows you to use the @Id annotation directly on the key fields.

<code class="java">@Entity
@IdClass(MyKey.class)
public class YourEntity {

    @Id
    private int id;

    @Id
    private int version;

    // Getters and setters
}</code>

<code class="java">public class MyKey implements Serializable {

    private int id;
    private int version;

    // Getters and setters
}</code>

Duplicating Entities:

To duplicate an entity with a different version, create a new instance with the same id but a different version value. This will create a separate entry in the database representing the new version.

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