How To Count Associated Entities Without Fetching Them In Entity Framework

Question

I\'ve been wondering about this one for a while now, so I thought it would be worth using my first Stack Overflow post to ask about it.

Imagine I have a discussion with

Answer 1

Easy; just project onto a POCO (or anonymous) type:

var q = from d in Model.Discussions
        select new DiscussionPresentation
        {
            Subject = d.Subject,
            MessageCount = d.Messages.Count(),
        };

When you look at the generated SQL, you'll see that the Count() is done by the DB server.

Note that this works in both EF 1 and EF 4.