How to conntect those two Entities - entity-framework

Question

I got a few entities and relationships in my application, which work perfectly fine. But there is a relationship, which I can't get right.
Scenario: I have Registrations and Professors. Each Registration has exactly two Professors assigned. But Professors can be assigned to multiple Registrations.
My Models:
public class ThesisRegistration : Entity
{
...
[Required] public long FirstExaminerId { get; set; }
public Professor FirstExaminer { get; set; }
[Required] public long SecondExaminerId { get; set; }
public Professor SecondExaminer { get; set; }
public List<ProfessorThesisRegistration> ProfessorThesisRegistrations { get; set; }
}
public class Professor : Entity
{
...
public List<ProfessorThesisRegistration> ProfessorThesisRegistrations { get; set; }
}
public class ProfessorThesisRegistration
{
public int ProfessorId { get; set; }
public Professor Professor { get; set; }
public int ThesisRegistrationId { get; set; }
public ThesisRegistration ThesisRegistration { get; set; }
}
Then the configuration:
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
...
modelBuilder.Entity<ProfessorThesisRegistration>().HasKey(i => new {i.ProfessorId, i.ThesisRegistrationId});
modelBuilder.Entity<ProfessorThesisRegistration>()
.HasOne(pt => pt.Professor)
.WithMany(f => f.ProfessorThesisRegistrations)
.HasForeignKey(pt => pt.ProfessorId);
modelBuilder.Entity<ProfessorThesisRegistration>()
.HasOne(pt => pt.ThesisRegistration)
.WithMany(t => t.ProfessorThesisRegistrations)
.HasForeignKey(pt => pt.ThesisRegistrationId);
}
What do I need to change, so this relationship works? Thank you!

Answer 1

If I wanna go straight to the point I should say that You are completely wrong in designing your database because, with that scenario that you define, the relation between Registrations and Professors is not Many-To-Many, rather it is a One-To-Many relation.
P.S: For Being tangible I add some property to your model.
so for resolving your problem, act like below:
public class ThesisRegistration
{
public int Id { get; set; }
public string Title { get; set; }
public int FirstExaminerId { get; set; }
public virtual Professor FirstExaminer { get; set; }
public int SecondExaminerId { get; set; }
public virtual Professor SecondExaminer { get; set; }
}
and change Professor Model like below:
public class Professor
{
public int Id { get; set; }
public string Name { get; set; }
public virtual ICollection<ThesisRegistration> ThesisRegistrationsOfFirstExaminer { get; set; }
public virtual ICollection<ThesisRegistration> ThesisRegistrationsOfSecondExaminer { get; set; }
}
and in the OnModelCreating Method:
modelBuilder.Entity<ThesisRegistration>()
.HasOne(p => p.FirstExaminer)
.WithMany(p => p.ThesisRegistrationsOfFirstExaminer)
.HasForeignKey(p => p.FirstExaminerId)
.OnDelete(DeleteBehavior.NoAction);
modelBuilder.Entity<ThesisRegistration>()
.HasOne(p => p.SecondExaminer)
.WithMany(p => p.ThesisRegistrationsOfSecondExaminer)
.HasForeignKey(p => p.SecondExaminerId)
.OnDelete(DeleteBehavior.NoAction);
now with this design, every Thesis can have exactly two examiners, and each professor can have multiple theses.
feel free to ask questions. good luck.