PhantomType で DB 保存前後の nullable な id を撲滅しつつオブジェクトのプロパティは共通化する

PhantomType.kt

sealed class State {
    object NotSaved : State()
    object Saved : State()
}

sealed class SessionDataModel<S : State> {
    abstract val title: String
    abstract val isLiked: Boolean

    fun toggleLike() {
        isLiked != isLiked
    }

    class SessionRawDataModel(
        override val title: String,
        override val isLiked: Boolean
    ) : SessionDataModel<State.NotSaved>() {
        fun save(id: SessionId): SessionSavedDataModel = SessionSavedDataModel(id, title, isLiked)
    }

    class SessionSavedDataModel(
        val id: SessionId,
        override val title: String,
        override val isLiked: Boolean
    ) : SessionDataModel<State.Saved>()
}

// application layer
fun save(sessionRawData: SessionDataModel.SessionRawDataModel) {
    val id = insertToDB(sessionRawData)
    val sessionSavedData: SessionDataModel.SessionSavedDataModel = sessionRawData.save(id)
}

// infra layer
fun insertToDB(sessionRawData: SessionDataModel.SessionRawDataModel): SessionId = TODO("DB にインサートして last insert id を返す")