Multi-Tenant and EF6 Design - entity-framework

Question

I'm designing a multi tenant website using EF6 code first, MVC, and other from the MS stack.
I want to have announcements for each Tenant. Simple enough, my EF code first class would look something like this:
class Announcement
{
public Announcement()
{
DateCreated = DateTime.Now;
}
[Key]
public int Id { get; set; }
public DateTime DateCreated { get; set; }
public string Title { get; set; }
public string Message { get; set; }
public virtual Tenant Tenant { get; set; }
public ApplicationUser Author { get; set; }
}
My design question is what if I want the site administrator to have the ability to post announcements to all tenants?
Since the database will force the Tenant relationship, I can't set the Tenant property to something artificial.
Before EF, I'd do something like this, but now I will lose the nice-to-have EF Navigation property.
class Announcement
{
public Announcement()
{
DateCreated = DateTime.Now;
}
[Key]
public int Id { get; set; }
public DateTime DateCreated { get; set; }
public string Title { get; set; }
public string Message { get; set; }
public int TenantID { get; set; }
public ApplicationUser Author { get; set; }
}
I would use TenantID appropriately for all Tenant based announcements, but would set it to 0 for site wide announcements.
So, is there a better design (other than two classes/tables) that can still leverage the EF Navigation properties?

Answer 1

What is the issue with
class Announcement
{
public Announcement()
{
DateCreated = DateTime.Now;
}
[Key]
public virtual int Id { get; set; }
public virtual DateTime DateCreated { get; set; }
public virtual string Title { get; set; }
public virtual string Message { get; set; }
public virtual int TenantID { get; set; } // during insert/Update set as required. the tenant should exist.
// A dummy SYSTEM wide tenant may be an option to consider
// navigation props
[ForeignKey("TenantID")]
public virtual Tenant Tenant { get; set; } // <<<<< NAV as before
public virtual ApplicationUser Author { get; set; }
}