Foreign key with OR logic - entity-framework

Question

I have CodeFirst design like this:
public class Email
{
public string Address { get; set; }
public Company Company { get; set; }
public int? CompanyId { get; set; }
public User User { get; set; }
public int? UserId { get; set; }
//many other props
}
public class Company
{
public List<Email> Emails { get; set; }
}
public class User
{
public List<Email> Emails { get; set; }
}
In a good way, Email can belong to only one foreign key: CompanyId or UserId. But now it allows CompanyId and UserId. It's wrong. Anyway, that design with nullables is ugly. For example, to get all emails linked to companies I need do this:
var companyEmails = _context.Emails.Where(x => x.CompanyId.HasValue);
I feel there is a better approach to define multiply foreign keys with OR logic. Please, help me find a way.

Answer 1

If you want to have the only one reference to Company or User, that mean it will not be FK and also you should have additional field with description, to which table this field points.
Alternatively, you can try Table per Hierarchy approach. At this case database table will remain almost the same, only new Discriminator column will be implicitly added, to distinguish classes, but you will can to write more "elegant" code:
public abstract class Email
{
public string Address { get; set; }
}
public class CompanyEmail : Email
{
public Company Company { get; set; }
public int? CompanyId { get; set; }
}
public class UserEmail : BaseEmail
{
public User User { get; set; }
public int? UserId { get; set; }
}
Usage:
var companyEmails = _context.Emails.OfType<CompanyEmail>();
//underlying query:
//select * from dbo.Emails where Discriminator = 'CompanyEmail'