Updating disconnected entities with many-to-many relationships

Question

Suppose I have the following model classes in an Entity Framework Code-First setup:

public class Person
{
    public int Id { get; set; }
    public string Name { get; set; }

    public virtual ICollection<Team> Teams { get; set; }
}

public class Team
{
    public int Id { get; set; }
    public string Name { get; set; }

    public virtual ICollection<Person> People { get; set; }
}

The database created from this code includes a TeamPersons table, representing the many-to-many relationship between people and teams.

Now suppose I have a disconnected Person object (not a proxy, and not yet attached to a context) whose Teams collection contains one or more disconnected Team objects, all of which represent Teams already in the database. An object such as would be created by the following, just for example, if a Person with Id 1 and a Team with Id 3 already existed in the db:

var person = new Person
{
    Id = 1,
    Name = "Bob",
    Teams = new HashSet<Team>
    {
        new Team { Id = 3, Name = "C Team"}
    }
};

What is the best way of updating this object, so that after the update the TeamPersons table contains a single row for Bob, linking him to C Team ? I've tried the obvious:

using (var context = new TestContext())
{
    context.Entry(person).State = EntityState.Modified;
    context.SaveChanges();
}

but the Teams collection is just ignored by this. I've also tried various other things, but nothing seems to do exactly what I'm after here. Thanks for any help.

EDIT:

So I get that I could fetch both the Person and the Team[s] from the db, update them and then commit changes:

using (var context = new TestContext())
{
    var dbPerson = context.People.Find(person.Id);
    dbPerson.Name = person.Name;
    dbPerson.Teams.Clear();
    foreach (var id in person.Teams.Select(x => x.Id))
    {
        var team = context.Teams.Find(id);
        dbPerson.Teams.Add(team);
    }

    context.SaveChanges();
}

This is a pain if Person's a complicated entity, though. I know I could use Automapper or something to make things a bit easier, but still it seems a shame if there's no way of saving the original person object, rather than having to get a new one and copy all the properties over...

Answer 1

The general approach is to fetch the Team from the database and Add that to the Person's Teams collection. Setting EntityState.Modified only affects scalar properties, not navigation properties.