Entity framework. Quering parent entity with subclass property loaded

Question

There is a simple domain..

public abstract class UserComment
{
    public string Text { get; set; }
}

public class BlogComment : UserComment
{
    public Blog Blog { get; set; }
}

public class PhotoComment : UserComment
{
    public Photo Photo { get; set; }
}

Is there way to query all entities of type UserComment with properties Blog and Photo loaded?

var comment = DbContext.Set<UserComment>()
    .Include(x => x.Blog) // will not compile
    .Include(x => x.Photo) // will not compile
    .FirstOrDefault();

if (comment is PhotoComment )
{
    string url = (comment as PhotoComment).Photo.Url;
}
if (comment is BlogComment)
{
    var dateCreated = (comment as BlogComment).Blog.DateCreated;
}

Thanks!

Answer 1

You will probably need two queries to get the result. If you only want the first element ( FirstOrDefault ) explicit loading - as you already proposed in the comments - is a good approach:

var comment = DbContext.Set<UserComment>().FirstOrDefault();

if (comment is BlogComment)
    DbContext.Entry(comment as BlogComment).Reference(bc => bc.Blog).Load();
else if (comment is PhotoComment)
    DbContext.Entry(comment as PhotoComment).Reference(pc => pc.Photo).Load();

If you want to load a list of UserComment s that's not the best solution as it would require to iterate over the loaded UserComment s and to call explicit loading for each element which will result in many queries.

For a list you can use the following approach that also will generate only two queries:

IEnumerable<UserComment> blogComments = DbContext.Set<UserComment>()
    .OfType<BlogComment>()
    .Include(bc => bc.Blog)
    .Cast<UserComment>()
    .AsEnumerable();

IEnumerable<UserComment> photoComments = DbContext.Set<UserComment>()
    .OfType<PhotoComment>()
    .Include(pc => pc.Photo)
    .Cast<UserComment>()
    .AsEnumerable();

List<UserComment> comments = blogComments.Concat(photoComments).ToList();

Because of the usage of AsEnumerable() this will run two separate database queries and concat the results to a single collection in memory.

LINQ-to-Entities supports Cast but for some reason it is not possible to remove the two AsEnumerable() conversions to get only a single database query and concat the results in the database. The code would still compile but I had a runtime exception about an invalid Include path.

I have tested with EF 4.1. It might be worth to test the query without AsEnumerable() with EF 5.0 to see if it still fails.