如何在kotlin中使用groupBy从复杂的结构中构造地图

Question

我对kotlin中的收藏感到困惑。

结构有点复杂。

data class Request(
    val region: Region,
    val promotions: List<Promotion>,
)


data class Promotion(
    var promotionCode: String,
    val promotionType: String,
    val coupons: List<Coupon>
) 


data class Coupon(
    val reminderType: ReminderType,
    val couponCode: String,
    val count: Int,
    val couponMultiLanguages: List<CouponDescription>
) 

现在我想得到一张地图，＜code＞地图

我被困在这里

val result: Map<ReminderType, List<Promotion>> = promotions.map { it.coupons.groupBy { it.reminderType } }

Answer 1

方法之一

val result = promotions.flatMap { p ->   //flatMap will flatten the list of list
    p.coupons.map { 
       it to p   // this creates a pair of coupon to promotion
    } 
}.groupBy(
   { it.first.reminderType } // specify reminderType as key selector
) { 
    it.second // promotion as value, groupBy adds these to list
}

在这里，我们首先将每个优惠券映射到促销，然后按优惠券分组