Latest modified row for each item - sql

Question

I have a sql table containing multiple rows with a memberid and lastmodified date. I need to get latest modified row for each member id. This is what I have tried in EFCore 3.1.1:
var a = context.Members
.Include(m => m.Histories.OrderByDescending(h => h.LastModifiedDate)
.FirstOrDefault());
and it gives error: Lambda expression used inside Include is not valid
What am I missing?
UPDATE:
I tried this as well that didn't work either:
var a = context.Histories
.GroupBy(h => h.MemberId)
.Select(g => g.OrderByDescending(p => p.LastModifiedDate).FirstOrDefault()).ToList();
The LINQ expression '(GroupByShaperExpression:
KeySelector: (o.MemberId),
ElementSelector:(EntityShaperExpression:
EntityType: History
ValueBufferExpression:
(ProjectionBindingExpression: EmptyProjectionMember)
IsNullable: False
)
)
.OrderByDescending(p => p.LastModifiedDate)' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to either AsEnumerable(), AsAsyncEnumerable(), ToList(), or ToListAsync(). See https://go.microsoft.com/fwlink/?linkid=2101038 for more information.

Answer 1

While newer versions of EF Core do support some filtering via adding Where clauses, it's best to think of entities as a "complete" or "complete-able" representation of your data state. You either use the complete state of data, or you project to get the details you want.
For example, if you just want the last modified date for each member:
var lastModifiedDetails = context.Members
.Select(m => new
{
m.MemberId,
LastModifiedDate = m.Histories.OrderByDescending(h => h.LastModifiedDate)
.Select(h => h.LastModifiedDate)
.FirstOrDefault()
}).ToList();
This would give you a collection containing the MemberId and it's LastModifiedDate.
Alternatively, if you want the complete member and a quick reference to the last modified date:
var memberDetails = context.Members
.Select(m => new
{
Member = m,
LastModifiedHistory = m.Histories.OrderByDescending(h => h.LastModifiedDate)
.FirstOrDefault()
}).ToList();
Here instead of trying to get the last modified date or the latest history through a Member entity, you get a set of anonymous types that project down that detail. You iterate through that collection and can get the applicable history and/or modified date for the associated Member.