如何在RavenDb MapReduce中执行MaxBy

使用RavenDB教程中的

Northwind

数据库，我试图按员工对订单进行分组，并为每个员工获取最新的订单

地图：

使用不存在的MaxBy进行Reduce：

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

视为适当的

DateTime

值，并正确地对其进行排序吗？

您需要像这样做

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,
}

---

1谢谢你，这很有效。2现在我看到Reduce产生了错误，但是我不理解原因。

TempMostRecent

是一个

IComperable

，我认为它应该是一个

系统。DateTime

，为什么它是一个

IComperable

？在服务器端，日期是作为字符串保存的，也许这是相关的？

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,
}