如何将三个列表连接到一个对象中

Question

如何在单个对象中根据 userId 转换三个列表，其中每个对象都将尊重的列表对象作为变量。

data class User(val id: String)
data class Book(val id: String, val userId: String)
data class Order(val id: String, val userId: String)

输入

val users: List<User> = listOf(
    User(id = "u1"),
    User(id = "u2"),
    User(id = "u3")
)
val books: List<Book> = listOf(
    Book(id = "b1.u1", userId = "u1"),
    Book(id = "b2.u1", userId = "u1"),
    Book(id = "b3.u2", userId = "u2"),
    Book(id = "b4.ux", userId = "ux")
)

val order: List<Order> = listOf(
    Order(id = "o1", userId = "u1"),
    Order(id = "o2", userId = "u1"),
    Order(id = "03", userId = "u2"),
    Order(id = "o4", userId = "u1")
)

输出

val result = listOf(Result(user, book, order))

Answer 1

您可以通过 userId 对书籍和订单进行分组，然后为每个用户选择相应的书籍和订单。

val booksMap = books.groupBy { it.userId }
val ordersMap = orders.groupBy { it.userId }
users.map {
    Result(
        user = it,
        books = booksMap[it.id] ?: emptyList(),
        orders = ordersMap[it.id] ?: emptyList()
    )
}

这里Result 是：

data class Result(val user: User, val books: List<Book>, val orders: List<Order>)

Answer 2

很难得到你所需要的输出。对我来说，为每个用户获取书籍和订单列表是最有意义的。如果是这种情况，那么您可以这样做：

data class Result(val user: User, val books: List<Book>, val orders: List<Order>)

val results = users.map { user ->
    val userBooks = books.filter { it.userId == user.id }
    val userOrders = orders.filter { it.userId == user.id }
    Result(user, userBooks, userOrders)
}