The right way to use one to many relationship in jdbi with kotlin

ContactPhonesTest.kt

import org.jdbi.v3.core.kotlin.KotlinMapper
import org.jdbi.v3.core.mapper.RowMapperFactory
import org.junit.jupiter.api.Test
import tj.alif.core.app.db.RepositoryTest
import org.jdbi.v3.core.result.RowView


data class Contact (
        val id: Int,
        val name: String,
        var phones: MutableList<Phone> = mutableListOf()
) {
    fun addPhone(phone: Phone) = phones.add(phone)
}

data class Phone (val id: Int,
             val type: String,
             val number: String
)

fun factory(type: Class<*>, prefix: String): RowMapperFactory {
    return RowMapperFactory.of(type, KotlinMapper(type, prefix))
}

class ContactPhonesTest: RepositoryTest() {

    @Test
    fun `some test`() {
        val SELECT_ALL = ("select contacts.id c_id, name c_name, "
                + "phones.id p_id, type p_type, phones.number p_number "
                + "from contacts left join phones on contacts.id = phones.contact_id "
                + "order by c_name, p_type")

        val contacts = db.createQuery(SELECT_ALL)
                .registerRowMapper(factory(Contact::class.java, "c"))
                .registerRowMapper(factory(Phone::class.java, "p"))
                .reduceRows(linkedMapOf()) { map: LinkedHashMap<Int, Contact>, rowView: RowView ->
                    val contact = map.computeIfAbsent(rowView.getColumn("c_id", Int::class.javaObjectType)) {
                        rowView.getRow(Contact::class.java)
                    }

                    if (rowView.getColumn("p_id", Int::class.javaObjectType) != null) {
                        contact.addPhone(rowView.getRow(Phone::class.java))
                    }
                    map
                }
                .toList().map { it.second }

        println(contacts)
    }
}