Question

I have a user class in EF Code First that contains a lot of properties, and each user has a collection of “Contacts” which are other users as a subset of the total user population. The other collection “ContactOfOthers” is just the reverse showing who has this user as a contact as this is a many-to-many relationship.

public class User {

        [Key]
        public string UserName { get; set; }

        // Lots of other properties not relevant to this question...

        [NotMapped]
        public bool IsMyContact { get; set; }

        public virtual ICollection<User> Contacts { get; set; }
        public virtual ICollection<User> ContactOfOthers { get; set; }

}

I introduced a not-mapped (not mapped to DB) property called IsMyContact. This is for cases when the user queries for a bunch of users and I need to show in the View which users are already in their contacts list. So this property should be true if the User is part of their “Contacts” collection. It shouldn’t be saved to the DB since it can be different for the same user, depending on the user doing the query.

Is there a nice way to do this in a query from the context? It could of course be brute-forced by doing two queries then iterating through the main one, looking for matches to the user’s Contacts collection, but I’m wondering if there’s a more elegant way to do this from one query, projecting a run-time computed column onto this property?

Answer 1

I don’t know a way how to populate the IsMyContact property in the User directly within the query. But an alternative approach could be to introduce a ViewModel which wraps the User and has in addition the IsMyContact flag:

public class UserViewModel
{
    public bool IsMyContact { get; set; }
    public User User { get; set; }
}

(The class User would not have the IsMyContact flag anymore.)

You could then project into this type when you run your query:

string me = "That's me"; // name of the user who is selecting

List<UserViewModel> list = context.Users
   .Where(u => ...some filter...)
   .Select(u => new UserViewModel
          {
              IsMyContact = u.ContactOfOthers.Any(c => c.UserName == me),
              User = u
          })
   .ToList();

The benefits would be: You need only one round trip and you are not forced to load the whole collection of Contacts to determine the IsMyContactFlag (but you can if you want to).

The drawback: You need this additional ViewModel type.