如何在 RavenDb MapReduce 中执行 MaxBy

Question

使用 RavenDB 教程中的 Northwind 数据库，我正在尝试按员工对订单进行分组，并为每位员工获取最近发送的订单。

地图：

from order in docs.Orders
select new {
    Employee = order.Employee,
    Count = 1,
    MostRecent = order.OrderedAt,
    MostRecentOrderId = order.Id
}

使用不存在的 MaxBy 减少：

from result in results
group result by result.Employee into grp
select new {
    Employee = grp.Key,
    Count = grp.Sum(result => result.Count),
    MostRecent = grp.Max(result => result.MostRecent),
    MostRecentOrderId = grp.MaxBy(result => result.MostRecent).MostRecentOrderId,
}

减少尝试：

from result in results
group result by result.Employee into grp
let TempMostRecent = grp.Max(result => result.MostRecent)
select new {
    Employee = grp.Key,
    Count = grp.Sum(result => result.Count),
    MostRecent = TempMostRecent,
    MostRecentOrderId = grp.First(result => result.MostRecent == TempMostRecent).MostRecentOrderId
}

但是我的 reduce 尝试返回 0 个结果。

另外：RavenDB 会将Order.OrderetAt 视为正确的DateTime 值并正确排序吗？

Answer 1

你需要这样做

from order in docs.Orders
select new {
    Employee = order.Employee,
    Count = 1,
    MostRecent = order.OrderedAt,
    MostRecentOrderId = order.Id
}

from result in results
group result by result.Employee into grp
let maxOrder = grp.OrderByDescending(x=>x.MostRecent).First()
select new {
    Employee = grp.Key,
    Count = grp.Sum(result => result.Count),
    MostRecent = maxOrder.MostRecent,
    MostRecentOrderId = maxOrder.MostRecentOrderId,
}