Question

I’ve got a standard social networking paradigm where a User has a collection of friends who are also users.

I’m using Entity Framwork Code First, and my Friend Relationship is defined as follows:

modelBuilder.Entity<User>()
.HasMany(u => u.Friends)
.WithMany()
.Map(m =>
    {
        m.ToTable("Friendships");
        m.MapLeftKey("UserId");
        m.MapRightKey("FriendId");
    });

What I want to do is to search my users table returning all users with an indicator of whether each returned user is friends with the current user. To be clear, I want to return Users who are Friends and Users who are not friends, but also with a boolean indicating whether each user is a friend. I know how to do this in TSQL its a basic left outer join.

I’ve seen examples of how to do a left join in LINQ, but all the examples I’ve seen are joining to a mapped type. My Friendships column doesn’t have a Mapped type.

How do I do this in EntityFramework?

Answer 1

var list = context.Users
    .Where(u => u.Age >= 20) // sample condition, if you want ALL remove the line
    .Select(u => new
    {
        User = u,
        FriendOfCurrentUser = u.Friends.Any(f => f.UserId == currentUserId)
    })
    .ToList();

Result is a list of anonymous objects containing the user and the boolean friendship indicator. You can also create a helper class UserWithFriendship and project into it (Select(u => new UserWithFriendship { ... }) instead of an anonymous type.