Error projecting from a grouped linq query

I have a database table that stores the following data regarding images:

public class EventImage
{
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int EventImageId { get; set; }
public string EventImageFileExtension { get; set; }
// Provided based on comments, but not used to reduce wasted bandwidth
public virtual byte[]? EventImageFullSizeBytes { get; set; }
// Provided based on comments, but not used to reduce wasted bandwidth
public virtual byte[]? EventImageThumbnailBytes { get; set; }
public int EventItemId { get; set; }
}

A typical DbSet is then used for querying:

public class WebAppContext : IdentityDbContext<WebAppRazorUser, ApplicationRole, int>
{
public DbSet<EventImage> EventImages { get; set; }
}

My goal is to retrieve one image per (parent) event item but without the binary data being included. Using different answers on SO I've pieced together the following:

var eventImages = context.EventImages.AsNoTracking()
// Filter to specific parent Id's
.Where(ei => uniqueEventIds.Contains(ei.EventItemId))
// Get only one image per parent
.GroupBy(img => img.EventItemId, (key, g) => g.OrderBy(img => img.EventImageId).FirstOrDefault()) 
// Project the image fields without the binary data
.Select(img => new 
{
img.EventImageId,
img.EventItemId,
img.EventImageFileExtension,
})
.ToList();

However its throwing an exception that I do not understand:

> KeyNotFoundException: The given key 'EmptyProjectionMember' was not present in the dictionary.
>
> System.Collections.Generic.Dictionary<TKey, TValue>.get_Item(TKey key)

If I remove the entire projection (.Select(...)) aspect, the first part of the query does return two images with all the fields populated, so I think that part is correct. I'm not sure what the error is relating to though.

Update

Swapping out certain elements provides strange results.

.Select(img => new object()) // Runs without an exception, but obviously doesn't actually give me what I need
.Select(img => new { }) // As above
.Select(img => new { Field1 = "Foo" }) // This fails with the same exception

Update 2

If I create a list from the initial search and then project from that, it appears to work:

var eventImages = (context.EventImages.AsNoTracking()
.Where(ei => uniqueEventIds.Contains(ei.EventItemId))
.GroupBy(img => img.EventItemId, (key, g) => g.OrderBy(img => img.EventImageId).FirstOrDefault()))
.ToList() // Fixes the issue
.Select(img => new 
{
img.EventImageId,
img.EventItemId,
img.EventImageFileExtension,
});

Looking at the underlying class I believe the exception was caused by the property being virtual but would still appreciate an authoritative answer please.

EF Core 6.0 / 7.0 query translation defect. But we should be tolerant, since translation of all LINQ query expression tree variations is a very complicated thing (if you try to do that yourself), so bugs/defects like this happen.

The workaround is to move the projection before the FirstOrDefault call (inside the GroupBy result selector), e.g.

.GroupBy(img => img.EventItemId, (key, g) => g
    .OrderBy(img => img.EventImageId)
    .Select(img => new 
    {
        img.EventImageId,
        img.EventItemId,
        img.EventImageFileExtension,
    })
    .FirstOrDefault()
)

Maybe your .net is old version. For your code I tried on .net7 efcore 7 it works fine

Try this to see if it works

           var eventImages = context.EventImages.AsNoTracking()
           .Where(ei => uniqueEventIds.Contains(ei.EventItemId))
           .GroupBy(img => img.EventItemId) 
           .Select(x => x.FirstOrDefault())
           .Select(img => new 
               {
                   img.EventImageId,
                   img.EventItemId,
                   img.EventImageFileExtension,
               }).ToList();