Question

Here is my user object:

public class User
{
    public int UserId { get; set; }
    public string Name { get; set; }
    public virtual ICollection<Tag> Tags { get; set; }
}

In my controller, I select the currently logged in user based on their user id:

User CurrentUser = Db.Users.Find((int)Session["UserId"]);

Next, I select a list of all users except the currently logged in user:

List<User> Users = Db.Users.Where(u => u.UserId != CurrentUser.UserId).ToList();

Here’s where it gets really confusing: I want to sort this list of users based on how closely their lists of tags compare to the currently logged in user’s list of tags. For instance, assume User A – who is currently logged in – has the following list of tags:

List<Tag> CurrentUserTags = new List<Tag>
{
    new Tag
    {
        TagId = 1
    },
    new Tag
    {
        TagId = 2
    },
    new Tag
    {
        TagId = 3
    }
}

User B has tags 1 and 2. User C only has tag 1. I want to sort the list of users as follows: User B then User C, because User B’s list of tags compares more closely to User A’s list of tags than User C’s list.

I really hope this makes sense, as I am completely stuck.

Answer 1

Make a hash set with the identities from the tags:

var currentTags = new HashSet<int>(CurrentUserTags.Select(t => t.TagId));

Then you can count the tags that exist in the hash set:

Users = Users.OrderByDescending(
  u => u.Tags.Count(t => currentTags.Contains(t.TagId))
).ToList();