How to define two FK in the same table? - asp.net

Question

I have a table called User which inherit the properties from IdentityUser, inside that table I added a reference to the UserFriendship table which need to store all the user friendship:
public class User : IdentityUser
{
public string FirstName { get; set; }
public DateTime BirthDate { get; set; }
public virtual ICollection<UserFriendship> UserFriendship { get; set; }
}
Essentially the UserFriendship contains two users, who are those who have a common friendship, this is the model definition:
public class UserFriendship
{
public int Id { get; set; }
[Key, ForeignKey("UserA")]
public string UserAId { get; set; }
public User UserA { get; set; }
[Key, ForeignKey("UserB")]
public string UserBId { get; set; }
public User UserB { get; set; }
[Required]
public DateTime DateTime { get; set; }
}
I defined the UserA and the UserB which are two FK of a User that are contained inside AspNetUsers table.
Now inside the FluentAPI I declared the following:
builder.Entity<UserFriendship>(entity =>
{
entity.HasKey(f => f.Id);
entity.HasOne(u => u.UserA)
.WithMany(n => n.UserFriendships)
.HasForeignKey(u => u.UserAId)
.IsRequired();
entity.HasOne(u => u.UserB)
.WithMany(n => n.UserFriendships)
.HasForeignKey(u => u.UserBId)
.IsRequired();
});
when I execute this command:
add-migration InitialMigration -context MyAppContext
I'll get:
Cannot create a relationship between 'User.UserFriendships' and 'UserFriendship.UserB', because there already is a relationship between 'User.UserFriendships' and 'UserFriendship.UserA'. Navigation properties can only participate in a single relationship.
I'm not an expert of EnityFramework, but based on that error I think that I cannot define two FK in the same table?
Sorry for any mistake, thanks.

Answer 1

You can define more than one FK in table.
The problem here is you are pointing two times to one navigation property - UserFriendships. The solution would be to create two navigation properties.
Navigation properties are used to browse the related data for specified foreign-key (you have one-to-many relationship) of entity.
Try this:
public class User
{
public string FirstName { get; set; }
public DateTime BirthDate { get; set; }
public ICollection<UserFriendship> UserAFriendships { get; set; }
public ICollection<UserFriendship> UserBFriendships { get; set; }
}
public class UserFriendship
{
public int Id { get; set; }
public string UserAId { get; set; }
public User UserA { get; set; }
public string UserBId { get; set; }
public User UserB { get; set; }
public DateTime DateTime { get; set; }
}
And define the relationship through fluent api as following:
modelBuilder.Entity<UserFriendship>(entity =>
{
entity.HasKey(f => f.Id);
entity.HasOne(u => u.UserA)
.WithMany(n => n.UserAFriendships)
.HasForeignKey(u => u.UserAId)
.IsRequired();
entity.HasOne(u => u.UserB)
.WithMany(n => n.UserBFriendships)
.HasForeignKey(u => u.UserBId)
.IsRequired();
});
What is more - you don't need to specify attributes Key, ForeignKey if you use Fluent API.