Entity Framework Core 将 DISTINCT 发送到生成的 SQL

Question

Is it possible to do send the DISTINCT keyword into the generated SQL command using Entity Framework Core? All the examples I see on grabbing distinct values is calling the .Distinct() after the values are already retuned, so the database is sending back a lot of data, when the database could do the distinct itself.

This would be for an anonymous object, with lots of values inside of it, from ints to strings and byte arrays.

I know I could do the .GroupBy().Select() , but again that is after the data has arrived, not before.

Example doesn't work with anonymous types:

(From a In DatabaseContext.Associate
 Join ac In DatabaseContext.AssociateClassification On ac.AssociateId Equals a.AssociateId
 Join ad In DatabaseContext.AssociateDepartment On ad.AssociateId Equals a.AssociateId
 Join ass In DatabaseContext.AssociateScheduleStaffing On ass.AssociateId Equals a.AssociateId
 Group Join ap In DatabaseContext.AssociatePhoto On ap.AssociateId Equals a.AssociateId Into apg = Group
 From ap In apg.DefaultIfEmpty()
 Where ac.ClassificationId.Value = ClassificationId AndAlso
       a.Inactive.Value = Convert.ToByte(0) AndAlso
       departments.Contains(ad.Department) AndAlso
       ass.UseJbDev.Value = Convert.ToByte(1) AndAlso
       Not DatabaseContext.Schedule.Any(Function(s) s.AssociateId = a.AssociateId AndAlso s.StartTime.Value >= Day AndAlso s.StartTime.Value < Day.AddDays(1))
 Select a.AssociateId, a.AssociateNumber, a.FullName, a.Inactive, ap.Photo, ass.DateSeniority).Distinct().ToList()

FYI: It does work however if I remove the ap.Photo and ass.DateSeniority and all objects then are the 'Associate' type, and Distinct works.

But again, it is after the fact, because when looking at the SQL created, it is not a SELECT DISTINCT, nor does the DISTINCT appear in the SQL.

Answer 1

Seems to me the answer is that "Distinct" extension method doesn't compare values of complex type objects. I need to implement IEqualityComparer<T> interface in order to compare the values of complex types.

That is the reason it worked fine if all properties of the object are from the same type, when you mix the properties and create an anonymous type, the built in compare doesn't work and need to implement your own comparer.