What is a composite key?

A composite (or concatenated) key in a database is a primary key consisting of two or more columns of a table. The uniqueness of a record is determined by the combination of values of all columns included in the composite key.

In Java applications using JPA, a composite key is usually implemented in one of two ways:

1. Using @Embeddable and @EmbeddedId:

   A separate class marked with the @Embeddable annotation is created, which encapsulates the fields that make up the composite key. In the entity, this class is embedded using the @EmbeddedId annotation. The class representing the composite key must implement Serializable and override the equals() and hashCode() methods.

   // Composite key class
   @Embeddable
   public class OrderItemId implements Serializable {
   
       private Long orderId;
       private Long productId;
   
       // Constructors, getters, setters
   
       @Override
       public boolean equals(Object o) {
           if (this == o) return true;
           if (o == null || getClass() != o.getClass()) return false;
           OrderItemId that = (OrderItemId) o;
           return Objects.equals(orderId, that.orderId) &&
                  Objects.equals(productId, that.productId);
       }
   
       @Override
       public int hashCode() {
           return Objects.hash(orderId, productId);
       }
   }
   
   // Entity with composite key
   @Entity
   public class OrderItem {
   
       @EmbeddedId
       private OrderItemId id;
   
       private int quantity;
   
       // Constructors, getters, setters
   }
   

2. Using @IdClass:

   A separate class representing the composite key (also implementing Serializable and overriding equals() and hashCode()) is created, and the entity specifies the name of this class using the @IdClass annotation. The fields that are part of the composite key are declared directly in the entity and marked with the @Id annotation. The field names in the entity and the key class must match.

   // Composite key class
   public class OrderItemId implements Serializable {
   
       private Long orderId;
       private Long productId;
   
       // Constructors, getters, setters
   
       @Override
       public boolean equals(Object o) {
           if (this == o) return true;
           if (o == null || getClass() != o.getClass()) return false;
           OrderItemId that = (OrderItemId) o;
           return Objects.equals(orderId, that.orderId) &&
                  Objects.equals(productId, that.productId);
       }
   
       @Override
       public int hashCode() {
           return Objects.hash(orderId, productId);
       }
   }
   
   // Entity with composite key
   @Entity
   @IdClass(OrderItemId.class)
   public class OrderItem {
   
       @Id
       private Long orderId;
   
       @Id
       private Long productId;
   
       private int quantity;
   
       // Constructors, getters, setters
   }