How to access discriminator column in JPA

Question

I have DisseminationArea as subcalss for Feature with the following code:

@Entity
@Table(name = "features")
@Inheritance(strategy = InheritanceType.JOINED)
@DiscriminatorColumn(name = "subtype_id", discriminatorType = DiscriminatorType.INTEGER)
public class Feature {

   @Id
   @Column(name="id")
   @GeneratedValue(generator="sqlite")
   @TableGenerator(name="sqlite", table="sqlite_sequence",
      pkColumnName="name", valueColumnName="seq",
      pkColumnValue="features")
   @Getter
   @Setter
   private long id;

   @ManyToOne
   @JoinColumn(name = "subtype_id")
   @Getter
   @Setter
   private FeatureSubtype featureSubtype;

   @ManyToOne
   @JoinColumn(name = "parent_id")
   @Getter
   @Setter
   private Feature parent;

   ...    
}

Unfortunately, this causes an exception when save this entity to database, because subtype_id field is used twice.

Can I annotate it somehow so that JPA know it is the same field?

Answer 1

If a discriminator column makes sense with InheritanceType.JOINED is worth discussing. I would tend to omit it on joined strategy and only use it with InheritanceType.SINGLE_TABLE.

Nevertheless, it's possible to map it without duplicate column errors.

If your superclass entity has an inheritance / discriminator definition like:

@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "subtype_id", discriminatorType = DiscriminatorType.INTEGER)

You just have to adjust the mapping to not update the value as usual by setting it readonly:

@Column(name="subtype_id", insertable = false, updatable = false)
protected int subTypeId;

public int getSubTypeId() {
    return subTypeId;
}

Now you can access the discriminator value of the entity.