One to many relationship via property delegation - Inspiration: https://github.com/jcornaz/xref

xref.kt

object OneToMany {

    private class One<O, M, C : MutableCollection<M>>(initial: O?, private val rightProperty: KMutableProperty1<O, C>) : ReadWriteProperty<M, O?> {
        var one: O? = initial

        override fun getValue(thisRef: M, property: KProperty<*>): O? = one

        override fun setValue(thisRef: M, property: KProperty<*>, value: O?) {
            when {
                value != null -> add(value, thisRef)
                else -> one?.let { remove(it, thisRef) }
            }
            one = value
        }

        private fun add(one: O, m: M) {
            (rightProperty.getDelegate(one) as Many<O, M, C>).many.add(m)
        }

        private fun remove(one: O, m: M) {
            (rightProperty.getDelegate(one) as Many<O, M, C>).many.remove(m)
        }
    }

    // todo(bnorm) - need to intercept add and remove events from the collection
    private class Many<in O, M, C : MutableCollection<M>>(initial: C, private val leftProperty: KMutableProperty1<M, O?>) : ReadWriteProperty<O, C> {
        var many = initial

        override fun getValue(thisRef: O, property: KProperty<*>): C = many

        override fun setValue(thisRef: O, property: KProperty<*>, value: C) {
            for (m in many) set(m, null)
            for (m in value) set(m, thisRef)
            many = value
        }

        private fun set(many: M, one: O?) {
            (leftProperty.getDelegate(many) as One<O, M, C>).one = one
        }

    }

    fun <O, M, C : MutableCollection<M>> many(initial: C, leftProperty: KMutableProperty1<M, O?>): ReadWriteProperty<O, C> {
        return Many(initial, leftProperty)
    }

    fun <O, M, C : MutableCollection<M>> one(initial: O?, rightProperty: KMutableProperty1<O, C>): ReadWriteProperty<M, O?> {
        return One(initial, rightProperty)
    }
}

class Parent {
    var children: MutableSet<Child> by OneToMany.many(mutableSetOf(), Child::parent)
}

class Child {
    var parent: Parent? by OneToMany.one(null, Parent::children)
}