Using Kotlin "data class" in bi-directional JPA relations

Data classes in Kotlin provide an easy way to implement entity beans for JPA without a lot of boilerplate code. This also includes toString(), hashCode() and equals() methods.

However when you are modelling a bi-directional relationship between to entities you run into trouble since the toString() methods of both entities will call each other recursively ending in a StackOverflowException eventually.

@Entity
data class Child(
        @Id @GeneratedValue
        val id: Long? = null,
        val name: String,
        @ManyToOne
        val parent: Parent? = null)

@Entity
data class Parent(
        @Id @GeneratedValue
        val id: Long? = null,
        val name: String,
        @ManyToOne(mappedBy = "parent")
        val children: MutableList<Child>? = null)

Solving this problem is easy. Since you can overwrite the generated toString() method with your own implementation you can do so handling this special case e.g. by not calling toString() on the parent class in the `toString() method of the child class. Alternatively you can print a single property of the parent or any other value

@Entity
data class Child(
        @Id @GeneratedValue
        val id: Long? = null,
        val name: String,
        @ManyToOne
        val parent: Parent? = null) {

    override fun toString(): String {
        return "Child(id=$id, name='$name', parent=${parent?.name})"
    }
}