How do you add new child entities to a parent in EF4?
// Objects created by POCO template generator
public class User
{
    public int Id { get; set; }
    public string Username { get; set; }
    public ICollection<Company> Companies { get; set; }
}
public class Company
{
    public int Id { get; set; }
    public string Name { get; set; }
    public ICollection<User> Users { get; set; }
}

public UserRepository
{
    internal ObjectContextEntities _dbContext = new ObjectContextEntities();
    public User Save(User pocoObject)
    {
        // Load the existing User which has no companies
        var loadedEntity = _dbContext.Users.Where(m => m.Id == pocoObject.Id).FirstOrDefault();

        // Add the new company from a POCO object
        loadedEntity.Companies.Add(pocoObject.Companies.Last());

        _dbContext.SaveChanges();
    }
}

public class User
{
    public int Id { get; set; }
    public string Username { get; set; }
    public ICollection<Company> Companies { get; set; }

    public User()
    {
        Companies = new List<Company>();
    }
}

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

public class POCOEntitiesContext : ObjectContext
{
    private ObjectSet<User> users;
    public ObjectSet<User> Users
    {
        get { return users; }
    }

    private ObjectSet<Company> companies;
    public ObjectSet<Company> Companies
    {
        get { return companies; }
    }

    public POCOEntitiesContext() : base("name=POCOEntities", "POCOEntitiesContainer")
    {
        users = CreateObjectSet<User>();
        companies = CreateObjectSet<Company>();
    }
}

using (var ctx = new POCOEntitiesContext())
{
    var loadedUser = ctx.Users.Where(u => u.Username == "Peri").First();
    loadedUser.Companies.Add(new Company() { Name = "New Company" });
    ctx.SaveChanges();
}

var newCompany = pocoObject.Companies.Last();
    newCompany.Users = null;
    loadedUser.Companies.Add(newCompany);

loadedUser.Companies.Add(new Company() { Name = "New Company" });