Can I use inline value class for mapping an Entity...

# spring

[Pihentagy]

Can I use inline value class for mapping an Entity field?

[Johan]

For spring-jdbc I had to add a converter for the Id field, but I think other fields worked without the converter but not sure.

[Pihentagy]

I planned to use it for an Id field. How should a converter look?

[Johan]

I made it this way https://github.com/navikt/tilleggsstonader-sak/blob/main/src/main/kotlin/no/nav/tilleggsstonader/sak/infrastruktur/database/IdConverters.kt

[Johan]

https://docs.spring.io/spring-data/relational/reference/commons/custom-conversions.html

Not sure it's the same for jpa

[Pihentagy]

Turns out I need jakarta.persistence.AttributeConverter for that I guess

[Pihentagy]

thanks for the example

[Pihentagy]

Still struggling with converters. AttributeConverter and @Convert(converter = Converter::class) does not solve the issue.

My setup:

@JvmInline value class LicensePlate private constructor(val value: String) {
    companion object {
        fun fromString(s: String): LicensePlate = LicensePlate(s.replace("-", ""))
    }
}

@Converter
class LicensePlateConverter : AttributeConverter<LicensePlate?, String?> {
    override fun convertToDatabaseColumn(value: LicensePlate?): String? = value?.value

    override fun convertToEntityAttribute(value: String?): LicensePlate? = value?.let { LicensePlate.fromString(it) }
}

interface VehicleRepository : JpaRepository<Vehicle, LicensePlate> {
    fun deleteByRegnumNotIn(regnums: List<LicensePlate>)
}

@Entity(name = "vehicles")
class Vehicle(
    @Convert(converter = LicensePlateConverter::class)
    @Column(name = "_id", columnDefinition = "character varying(20)")
    @Id
    var regnum: LicensePlate,
 ...
)

org.springframework.orm.jpa.JpaSystemException: Could not convert 'xxx.personnel.LicensePlate' to 'java.lang.String' using 'org.hibernate.type.descriptor.java.StringJavaType' to wrap


during a call to

val vehicleList = vehicleRepo.findAllById(regnums)

[Johan]

Maybe one of these can help?
https://stackoverflow.com/questions/44069361/convert-on-id-field

[Pihentagy]

Hm, so I define my Id in the PK class, and what about the repository? Should it be Repository<Vehicle, String>, or Repository<Vehicle, LicensePlate>?

[Pihentagy]

@JvmInline
value class LicensePlate private constructor(
    val value: String) {
    companion object {
        fun fromString(s: String): LicensePlate = LicensePlate(s.replace("-", ""))
    }
}

data class VehiclePK(
    @Convert(converter = LicensePlateConverter::class)
    @Column(name = "_id", columnDefinition = "character varying(20)")
    val regnum: LicensePlate
) : Serializable

@Entity(name = "vehicles")
@IdClass(VehiclePK::class)
class Vehicle(
    @Id
    var regnum: LicensePlate,

interface VehicleRepository : JpaRepository<Vehicle, VehiclePK> {
    fun deleteByRegnumNotIn(regnums: List<LicensePlate>)
}

but still getting

org.springframework.orm.jpa.JpaSystemException: Error attempting to apply AttributeConverter: class java.lang.String cannot be cast to class hu.icontest.personnel.LicensePlate (java.lang.String is in module java.base of loader 'bootstrap'; xxx.personnel.LicensePlate is in unnamed module of loader 'app')

during a call of

val vehicleList = vehicleRepo.findAllById(regnums)

[Szymon Jeziorski]

let me ask you from the other side: Do you need this to be a value class? JPA is written to primarily work with Java classes, so I wouldn't be surprised for issues to arise when you use strictly Kotlin specific things like inline classes.
Also, if you're bringing in JPA and all its machinery, I'm pretty confident performance advantage of using inline class here is negligible.
I mean, if you're doing this for fun, than go for it, but if that's some real problem you're trying to solve, I would just go for normal data class (or even type alias or plain String if you don't have any other logic around LicensePlate class and just use it as an alias)

[Pihentagy]

From DDD point of view, it adds much, if it is not just a string, but wrapped in its own type.

[Pihentagy]

https://kotlinlang.org/docs/inline-classes.html#inline-classes-vs-type-aliases

[Szymon Jeziorski]

Sure, I'm not saying it doesn't add anything. That was just a general advice, of course it's context-dependent. Sometimes you might want to use String, other times value class, and other typealiases.
Now I've seen that you limit LicensePlate creation to custom factory function so value class might indeed be a good fit for it. Didn't notice that earlier.

That being said, I still think you might be better off using normal data class for this scenario, to save yourself a headache of fighting with JPA not being compatible with inline classes.

Also, looks like issue for this was raised on spring data commons Github, you might want to take a look at it:
https://github.com/spring-projects/spring-data-commons/issues/2868

[Pihentagy]

Well, understood. I should then use inline classes at another levels of the app, not directly on the Entities.