Question

I have a Profile class that has a father property which self references to itself, I want to load a profile together with its fathers profile in one sql join statement

Select * FROM profile left join profile as father on profile.fatherid = father.id where profile.id = 650

So i created the following linq statement, but instead of running the sql statement about it runs the following statements

select * from profiles

select * from profiles where id = 650

Then in memory it groups them together, but obviously i would not want to load the whole database.

    private class Result
    {
        public Profile Profile { get; set; }
        public IEnumerable<Profile> Fathers { get; set; }
    }
    private Result MapFather(Profile p, IEnumerable<Profile> father)
    {
        return new Result() {Profile = p, Fathers = father.DefaultIfEmpty()};
    }

   var profiles = from p in db.Profiles where p.ID.Equals(650) select p;
   var fathers = from f in db.Profiles select f;
   var groupJoin = profiles.GroupJoin(fathers,
                                        p => p.FatherID, 
                                        f => f.ID, 
                                        MapFather).ToList();

Answer 1

Your call to GroupJoin() is getting the IEnumerable version because of your call to MapFather is a Func<,> instead of Expression>. Therefore the calls to profiles and fathers are being executed before the GroupJoin.

Try inlining MapFather():

var groupJoin = profiles.GroupJoin(fathers,
                                    p => p.FatherID, 
                                    f => f.ID, 
                                    (p, fathers) => new Result() {Profile = p, Fathers = father.DefaultIfEmpty()}).ToList();