Question

I have written a Linq query to do an outer join on multiple columns. My problem is that I want to OR the columns. In other words, I need the join condition:

on Bar.Id equals Foo.BarId1 or Bar.Id equals Foo.BarId2

Some examples I have seen on this site have used anonymous types to create AND conditions, but I can’t figure out how to construct an OR using anonymous type or if it is even possible. I can find no example that matches.

My query:

    var data = (from f in Foos
                join b in Bars on f.BarId1 equals b.Id into tb
                  from xb in tb.DefaultIfEmpty()
                join b in Bars on f.BarId2 equals b.Id into tb2
                  from xb2 in tb2.DefaultIfEmpty()
                select new { Foo = f, Bar1 = xb, Bar2 = xb2 });

This works, but I have a feeling it isn’t the best solution

EDIT

I spoke too soon. It doesn’t work: when Foo.BarId1 points to a valid Bar, and Foo.BarId2 does not, it actually returns the same Bar for Bar1 and Bar2. I was expecting Bar1 to be valid and Bar2 to be null in this case.

EDIT

I’m getting closer. This is my latest query:

var data = from f in Foos
            from b in Bars.Where(x => x.Id == p.BarId1 || x.Id == p.BarId2).DefaultIfEmpty()
            select new { Foo = p, Bars = b };

I was expecting it to return:

Foo Bar    
f1  b1      
    b2

What I get is:

Foo Bar
f1  b1
f1  b2

EDIT

I’ve finally found the query that returns exactly what I want:

var data = from f in Foos
       select new
       {
           Foo = f,
           Bar1 = Bars.FirstOrDefault(x => x.Id == f.Bar1Id),
           Bar2 = Bars.FirstOrDefault(x => x.Id == f.Bar2Id)
       };

Would still like to know of any improvements I can make to this.

FINAL EDIT

I have gone back to the original query:

    var data = (from f in Foos
                join b in Bars on f.BarId1 equals b.Id into tb
                  from xb in tb.DefaultIfEmpty()
                join b in Bars on f.BarId2 equals b.Id into tb2
                  from xb2 in tb2.DefaultIfEmpty()
                select new { Foo = f, Bar1 = xb, Bar2 = xb2 });

It turns out that this was in fact working, but an error in my Linqpad test made me think it wasn’t.

It is more efficient than the previous one too – in SQL Profiler I can see that it generates 1 SQL select, compared to the previous, which generates 3*n selects (does 2 Bar selects for every Foo select)

Answer 1

It can be very simple if you are willing to introduce navigation properties Foo.Bar1 and Foo.Bar2. Then you can just do:

from f in Foos
select new { f, f.Bar1, f.Bar2 }

This will create outer joins in SQL. (Which may be a reason not to like these navigation properties, because it is easy to forget that they cause outer joins, without them you’re always urged to take control yourself).