ASP.NET Core connect two table model returns NullReferenceException

Question

I have a strange issue with model connection. I'm using ASP.NET Core with SQL Server and Entity Framework Core.

User model:

public class User
{
    [Key]
    public int Id { get; set; }
    [Required]
    public string Name { get; set; }
    [Required]
    public string Email { get; set; }
    [Required]
    public string PasswordHash { get; set; }    
    public ICollection<Order> Orders { get; set; }
}

Order model:

public class Order
{
    [Key]
    public int Id { get; set; }
    [Required]
    public string Test { get; set; }
    public int UserId { get; set; }
    public User User { get; set; }
}

Context:

public DbSet<User> Users { get; set; }

public DbSet<Order> Orders { get; set; }

public DatabaseContext(DbContextOptions options) : base(options)
{
    //
}

I get an error

System.NullReferenceException: 'Object reference not set to an instance of an object.'

when I'm trying to add orders to user with this code:

public void Test(User user)
{
    user.Orders.Add(new Order() { Test = "test" });
    _context.SaveChanges();
}

The only solution was this but the problem is that the ICollection<Order> Orders count 0 every time:

private ICollection<Order> _orders;
public virtual ICollection<Order> Orders
{
    get { return _orders ?? (_orders = new Collection<Order>()); }
    set { _orders = value; }
}

I tried to make ICollection<Order> and User virtual , but the error stays the same.

Thanks for your answers.

Answer 1

Remove collection initialisation from class and add order in the following way:

public void Test(User user)
{
    _context.Orders.Add(new Order() { Test = "test", User = user });
    _context.SaveChanges();
}

Answer 2

Try this:

public void Test(User user)
{
    _context.Orders.Add(new Order { Test = "test", UserId=user.Id});
    _context.SaveChanges();
}