class Order {
   @Id
   String id;                        // technically, randomly generated value without _any_ business meaning, e.g. UUID
   String reference ;           // some kind of business information encoded in the value and it is _unique_ (!)
   @MappedCollection
   Set<OrderItem> orderItems;
   // might be a better name compared to @JoinColumn
   //@MappedColumn
   @JoinColumn("reference")  // <== not using the "id" field
   private Meta meta;
}

class OrderItem {
   @Id
   Long id;                      // technical id
   String orderId;            // back reference to Order
}

class Meta {
   @Id
   String reference;
   String data;
}