[FIXED] How to handle shared column with jpa/hibernate?

Issue

I'm storing money/currency stored in the database which I want to map using JPA/Hibernate but one column is needed for two different mappings on the same entity.

DB Schema:

id: number
currency_code: string
total: number
sub_total: number

POJOs:

public class MyEntity {
    private long id;
    private Money total;
    private Money subTotal;
}

public class Money {
    private  CurrencyCode currencyCode;
    private  BigInteger value;
}

The problem I'm hitting with JPA/Hibernate is that the currency_code is needed to be mapped with both total and subTotal:

@Entity
@Table(name = "MyTable")
public class MyEntity {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private long id;

    @Type(type = "customMoneyType")
    @Columns(columns = {
            @Column(name = "currency_code"),
            @Column(name = "total"),
    })
    private Money total;

    @Type(type = "customMoneyType")
    @Columns(columns = {
            @Column(name = "currency_code"),
            @Column(name = "sub_total"),
    })
    private Money subTotal;
}

which leads to Caused by: org.hibernate.MappingException: Repeated column in mapping for entity

Both fields need to be updatable and nullable. The sharing of the column is legacy but not changeable in the near future. With that said, I'm open to less than ideal solutions which could work until the DB schema can be changed.

Solution

Just make MyEntity to align exactly to the table. Keep the currencyCode, total and subTotal fields as internal to the class and use OOP encapsulation technique to expose a methods for client work with them though the Money value object. Something like :

@Entity
@Table(name = "entity")
public class MyEntity {

    @Id
    @Column(name = "id")
    private long id;

    @Column(name = "currency_code")
    private CurrencyCode currenyCode;
    
    @Column(name = "total")
    private BigInteger total;

    @Column(name = "sub_total")
    private BigInteger subTotal;


    public Money getTotal(){
      return new Money(currenyCode, total);
    }

    public Money getSubTotal(){
      return new Money(currenyCode, subTotal);
    }

    public void updateTotal(Money m){
        this.currencyCode = m.getCurrenyCode();
        this.total = m.getValue();
    }

    public void updateSubTotal(Money m){
        this.currencyCode = m.getCurrenyCode();
        this.subTotal = m.getValue();
    }

}

Answered By - Ken Chan
Answer Checked By - Timothy Miller (JavaFixing Admin)