Net Core ApplicationUser with foreign key to itself - entity-framework

Question

I'm using Net Core 2.2 and Entity Framework. I have an ApplicationUser class used for multiple user types with foreign keys to each other but entity framework gives an error when adding a migration:
Unable to determine the relationship represented by navigation property 'ApplicationUser.Class' of type 'Class'. Either manually configure the relationship, or ignore this property using the '[NotMapped]' attribute or by using 'EntityTypeBuilder.Ignore' in 'OnModelCreating'.
I'm unsure what the proper way to implement this is. Can anyone help me?
My ApplicationUser class:
public class ApplicationUser : IdentityUser
{
public string CustomerId { get; set; }
public string TeacherId { get; set; }
public int? ClassId { get; set; }
public int? SettingsId { get; set; }
public virtual ApplicationUser Customer { get; set; }
public virtual ApplicationUser Teacher { get; set; }
public virtual Class Class { get; set; }
public virtual Settings Settings { get; set; }
public virtual ICollection<Class> Classes { get; set; }
public virtual ICollection<License> Licenses { get; set; }
public virtual ICollection<Exercise> Exercises { get; set; }
public virtual ICollection<GameProgress> GameProgresses { get; set; }
public virtual ICollection<StudentExercise> StudentExercises { get; set; }
}
And because it's in the error, my Class class:
public class Class
{
public int Id { get; set; }
public string TeacherId { get; set; }
public int? SettingsId { get; set; }
public virtual ApplicationUser Teacher { get; set; }
public virtual Settings Setting { get; set; }
public virtual ICollection<Exercise> Exercises { get; set; }
public virtual ICollection<ApplicationUser> Students { get; set; }
}

Answer 1

The problem has nothing to do with self-referential ApplicationUser relationships. The exception specifically tells you this is a problem with Class. The issue is that you've got a reference property to Class on ApplicationUser, but on Class, you've got both a collection of ApplicationUsers (Students) and a separate reference to ApplicationUser (Teacher). Simply, EF doesn't know which this foreign key is actually referring to.
The solution is exactly what the exception tells you: add fluent config to clear up the ambiguity:
public override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<ApplicationUser>()
.HasOne(x => x.Class)
.WithMany(x => x.Students);
}