从EF Core中的子实体获取最大值

Question

Let's say I have Companies which have Customers which have Orders. In one search query I want to set the order in a way that brings the "companies with latest orders first". (Hope this is clear enough).

.OrderByDescending(x =>
x.Customers.Max(c => c.Orders.Any() ? 
        y.Orders.Max(o => (DateTime?) o.DateCreatedUtc) 
        : DateTime.MinValue)
) 

• If customer has no order consider the latest order date as DateTime.MinValue

Currently I'm getting this warning: The LINQ expression 'Max()' could not be translated and will be evaluated locally

Can we rewrite this to be evaluated on the server?

Answer 1

Initially (my apologies) it looks exactly the same as using Max() in Orderby - make sure you use nullable overloads of outer (essentially every) Max / Min calls and you get translation.

Using nullable overloads is still a must. However, there is a hidden trap. - currently EF Core can translate aggregate methods only if they use simple member selector with optional cast. Any other expression is causing client evaluation regardless of the type being nullable or not.

Fortunately there is simple solution which works for all standard aggregates except Average - flatten the aggregate set using SelectMany (could be more than one and could be combined with reference navigation properties) to the level containing the aggregating entity and apply the aggregate function on that set.

Also make sure not using conditional or similar expressions inside the aggregate selector. The good thing about nullable Max / Min overloads is that it returns null for empty set, so there is no need to include Any checks. null s normally come first in order, so normally you don't need special sentinel values. And even if you do need them, make sure to apply them using C# ?? operator after the aggregate call.

With all that being said, the translatable version of the sample snippet is

.OrderByDescending(x => 
    x.Customers.SelectMany(c => c.Orders).Max(o => (DateTime?)o.DateCreatedUtc))
//                  ^                                  ^
//               drill down                    convert to nullable