JPA @one-to-many // rewrite stored ID

Question

I would like to rewrite the stored value for a @ManyToOne relationship before writing to the database to mimic the behaviour of some legacy code. Latter stores a unset relationship as 0 instead of null. All other values (>0) are regular references.

Is there a way to implement a custom mapping for the @ManyToOne value to handle this?

Entities:

@Entity
@Table(name = "Content")
public class ContentEntity
    // ..
    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name = "ColorID")
    private ColorEntity color;
}

@Entity
@Table(name = "Color")
public class ColorEntity {
    @Id
    @GeneratedValue
    @Column(name = "ID")
    private String id;
    // ..
}

Database:

    CREATE TABLE `Content` (
        # ..
        `ColorID` VARCHAR(255) DEFAULT NULL);
    CREATE TABLE `Color` (
        `ID` VARCHAR(255) PRIMARY KEY,
        # ..)

There is no corresponding entry in the target table with id:0 but multiple records for id:>0

I'm using JPA in a Spring Boot application.

user_412446 · Answer

I think you can use a column transformer for this purpose:

@ColumnTransformer(
    forColumn = "ColorID",
    read = "nullif( ColorID, 0 )",
    write = "coalesce( ?, 0 )"
)
@ManyToOne(fetch = FetchType.EAGER)
@JoinColumn(name = "ColorID")
private ColorEntity color;

JPA @one-to-many // rewrite stored ID

1 Answers