Example entity with composite key Spring Boot and Kotlin

Property.kt

// This will not serialize!
data class Muppet(val name: String)

// This will serialize!
data class Puppet(val name: String = "")

// Composite key class must implement Serializable
// and have defaults.
class PropertyId(
        val uuid: UUID = UUID.randomUUID(),
        val name: String = "") : Serializable

// Need defaults everywhere!
@Entity
@IdClass(PropertyId::class)
data class Property(
        @Id val uuid: UUID = UUID.randomUUID(),
        @Id val name: String = "",
        val value: String = "")

To add to what @Mineralf said, the Kotlin JPA plugin needs a small hint to create that no-arg constructor. I found that I needed to add @Embeddable to the data class to get it to work.

@Embeddable
data class PropertyId(
        val uuid: UUID,
        val name: String) : Serializable

Thanks a lot this was solving our problems!

Thanks a lot for pointing to @IdClass and @Id annotations!
I tried using @EmbeddedId first and couldn't get rid of "org.springframework.orm.jpa.JpaSystemException: Could not set field value [] value by reflection : " error.