DataContext always returns null as foreign key

we are currently working on a little ASP.NET Page. We're using EntityFramework (EF) and have created a BaseController which handles our DataContext.

Simplified we have 2 Models:

The User-Model (from automatic generated code):

public class User : IdentityUser
    {
        public virtual ICollection<Location> Locations { get; set; }
        public virtual ICollection<Calendar> Calendars { get; set; }

        public async Task<ClaimsIdentity> GenerateUserIdentityAsync(UserManager<User> manager)
        {
            // Note the authenticationType must match the one defined in CookieAuthenticationOptions.AuthenticationType
            var userIdentity = await manager.CreateIdentityAsync(this, DefaultAuthenticationTypes.ApplicationCookie);
            // Add custom user claims here
            return userIdentity;
        }
    }

And our own Calendar-Model:

 public class Calendar
    {
        [Key]
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public Guid Id { get; set; }

        [DataType(DataType.Text)]
        public string GoogleId { get; set; }

        [Required]
        [DataType(DataType.Text)]
        public string Title { get; set; }

        [DataType(DataType.Text)]
        public string Description { get; set; }

        public ICollection<Event> Events { get; set; }
        public User User { get; set; }
    }

So we have the connection between these models with: One User can have many Calendars. But only One Calendar can be from one User.

The following Code is our DataContext:

public class DatabaseContext : IdentityDbContext<User>
{
    public DatabaseContext()
    {
        // ReSharper disable once UnusedVariable
        var instance = SqlProviderServices.Instance;
    }

    public static DatabaseContext Create()
    {
        return new DatabaseContext();
    }

    public DbSet<Location> Locations { get; set; }

    public DbSet<Calendar> Calendars { get; set; }

    public DbSet<Event> Events { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);

        modelBuilder.Entity<User>().HasMany(c => c.Calendars);
    }
}

The Problem now is, when I use the DataContext I get everything out of the database. But

DataContext db = new DataContext()
db.Calendar ...

In Calendar the User is always null!

When I adding a Calendar to the Database with ASP.NET MVC in the following Code:

        public ActionResult Create([Bind(Include = "Title,Description")] Models.Calendar calendar)
        {
            if (ModelState.IsValid)
            {
                calendar.User = CurrentUser;
                calendar.Events = new List<Event>();
                Db.Calendars.Add(calendar);
                Db.SaveChanges();
                return RedirectToAction("Index");
            }

            return View(calendar);
        }

I debugged this Method and in the first line in the IF it really sets the right user into the calendar. Can anyone tell me, why Calendar.User is always null?