Why doesn't an entity refer to another entity that created it?

Question

ASP.NET

I have a Teacher entity that creates a course. The problem is that the course, for some reason, does not reference the teacher who created it. The teacher does not refer to the course either. I tried using the Update method, but it did not help.

public async Task<CourseResponse> AddCourse(CourseAddRequest courseAddRequest, Guid teacherId)
       {
            var teacher = await _teacherRepository.GetTeacher(teacherId);

            if (teacher == null)
            {
                throw new Exception("User doesn't exist");
            }
            var course = _mapper.Map<Course>(courseAddRequest);
            course.Teacher = teacher;
            course.TeacherId = teacher.Id;
            var addedCourse = await _courseRepository.AddCourse(course);
            teacher.Courses.Add(addedCourse);
            teacher = await _teacherRepository.UpdateTeacher(teacher);
            return _mapper.Map<CourseResponse>(addedCourse);
        }

teacher:

public class Teacher : BaseEntity
    {
        public string Name { get; set; }
        public string LastName { get; set; }
        public List<Course> Courses { get; set; } = new List<Course>();
    }

course:

public class Course : BaseEntity
    {
        public string Header { get; set; }
        public string Description { get; set; }
        public Guid TeacherId { get; set; }
        public Teacher Teacher { get; set; }

        public List<Student> StudentsOnCourse { get; set; } = new List<Student>();
    }

I tried to organize a one-to-many relationship, but it did not help:

class CourseEntityTypeConfiguration : IEntityTypeConfiguration<Course>
    {
        public void Configure(EntityTypeBuilder<Course> builder)
        {

            builder.HasOne(c => c.Teacher)
                .WithMany(t => t.Courses)
                .HasForeignKey(c => c.TeacherId);            
            
        }
    }

public class UniDbContext : DbContext
    {
        public UniDbContext(DbContextOptions<UniDbContext> options) : base(options)
        {

        }

        public DbSet<Course> Courses { get; set; }
        public DbSet<Student> Students { get; set; }
        public DbSet<Teacher> Teachers { get; set; }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.ApplyConfigurationsFromAssembly(typeof(CourseEntityTypeConfiguration).Assembly);


        }
    }

In startup:

services.AddDbContext<UniDbContext>(options =>
                options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection"),
                    sqlOpt => sqlOpt.MigrationsAssembly(typeof(UniDbContext).Assembly.FullName)
            ));

I have a git repository where you can see everything in more detail:
https://github.com/ChuvashPeople/UniversityProject
Thank you for your help.

Answer 1

Yes, you just need to write Include so that related entities are loaded. This will turn into a join on the sql side. By default, loading related entities is disabled.

public async Task<Teacher> GetTeacher(Guid teacherId)
        {
            var teacher = await _uniDbContext
                                        .Teachers
                                        .Include(x => x.Cources)
                                        .FirstOrDefaultAsync(t => t.Id == teacherId);
            return teacher;
        }