EF Loading Related Entities in Extension Methods - entity-framework

Question

With the below 1 to many relationship setup:
public partial class Folder
{
public Folder()
{
Files = new HashSet<File>();
}
public int FolderId { get; set; }
public virtual ICollection<File> Files { get; set; }
}
public partial class File
{
public int FileId { get; set; }
public int FolderId { get; set; }
public virtual Folder Folder { get; set; }
}
public class DataContext: DbContext
{
...
public virtual DbSet<Folder> Folders { get; set; }
public virtual DbSet<File> Files { get; set; }
...
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<Folder>(entity =>
{
...
});
modelBuilder.Entity<File>(entity =>
{
...
entity.HasOne(e => e.Folder)
.WithMany(e => e.Files)
.HasForeignKey(e => e.FolderId)
.OnDelete(DeleteBehavior.ClientSetNull)
.HasConstraintName("...");
});
}
}
public SomeReturnType AFunction(IQueryable<Folder> folders) =>
folders.GroupJoin(
...,
(folder, other) => new
{
folder,
something = folder.Files.Any(...)
});
With AFunction, the related Files are loaded for folder.Files.Any(). But if I do the below:
public static class FolderExtensions
{
public static SomeReturnType BFunction(this Folder folder) =>
folder.Files.Any(...);
}
public SomeReturnType AFunction(IQueryable<Folder> folders) =>
folders.GroupJoin(
...,
(folder, other) => new
{
folder,
something = folder.BFunction()
});
BFunction will not load the related Files. Any ideas to get around this problem?

Answer 1

To explain what happens let's first reduce the query to the essentials:
folders.Select(folder => new
{
folder,
something = folder.BFunction()
});
The problem here is that the BFunction method is executed client-side. That is, the part IQueryable<Folder> folders is first materialized into memory (client-side) and then the projection (new { ... }) is applied to the materialized result. This is something EF core does if anything in the final projection (and only in the final projection) can't be translated into SQL. Since Files are not Include-ed and no lazy loading occurs, the materialized folders don't have files and BFunction evaluates to false.
EF doesn't try to expand CLR functions into expression trees itself. In this case it would be possible, but often it won't be. At any rate, EF doesn't even try.
You have to do the expansion yourself:
folders.Select(folder => new
{
folder,
something = folder.Files.Any()
});
Including the files would also work, but of course that's more expensive:
folders
.Include(f => f.Files)
.Select(folder => new
{
folder,
something = folder.BFunction()
});
Side-note: the GroupJoin in this form is presently not supported by EF core (6.0).