How to do a MaxBy in RavenDb MapReduce - mapreduce

Question

Using the Northwind database from RavenDB tutorial I'm trying to group orders by employee and get the most resent order for every employee.
Map:
from order in docs.Orders
select new {
Employee = order.Employee,
Count = 1,
MostRecent = order.OrderedAt,
MostRecentOrderId = order.Id
}
Reduce with nonexisting MaxBy:
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,
}
Reduce attempt:
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
}
However my reduce attempt returns 0 results.
Also: will RavenDB treat the Order.OrderetAt as a proper DateTime value and order them correctly?

Answer 1

You need to do it like
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,
}