Cross model update in ASP.NET Core

Question

I'm really new to ASP.NET Core so I apologize if this is a silly question, but I've spent so many hours trying to figure this out.

I have 2 models, Teacher and Student . The relationship is one teacher to many students. This is a backend API project and Im working on a PUT method which can update fields in both models simultaneously in one request.

Here's my Teacher class:

public partial class Teacher
{
        public Teacher()
        {
            Students = new HashSet<Student>();            
        }

        public int TeacherId { get; set; }
        public string Name { get; set; }
        ... tons of other properties ...
}    

Here's my Student class:

public partial class Student
{       
        public int Id { get; set; }
        public int TeacherId { get; set; }
        public string Name { get; set; }
        public virtual Teacher Teacher { get; set; }        
}

Here's the controller:

[HttpPut("{id}")]
public async Task<IActionResult> PutTeachers(int id, TeacherViewModel model)
{            
    var result = await _service.UpdateAsync(model);          
    return Ok(result.Message);
}

(The code above is simplified) - It takes in a TeacherViewModel which restricts the number of fields to be returned - I used another class as a service to do the update

Here's the service class:

public class TeacherService
{
        private readonly Repository<Teacher> _repository;        

        public TeacherService(DatabaseContextWrapper context)
        {
            _repository = new Repository<Teacher>(context);            
        }

        public async Task<ITransactionResult> UpdateAsync(TeacherViewModel model)
        {
            var teacher = _repository.FindAsync(model.TeacherId).Result;

            teacher.TeacherId = model.TeacherId;
            teacher.Name = model.Name;           

            teacher.Students.Clear();

            foreach(var student in model.Students)
            {
                teacher.Students
                    .Add(new Student
                    {
                        Id = Student.Id,
                        TeacherId = Student.TeacherId
                        Name = Student.Name
                    });```
            }
        }
}

My reasoning is to add the Student model to the to students under the Teacher model but it doesn't iterate through. If I comment out the clear code, the update will work but it won't cross update. It just simply wont iterate through. I guess I'm pretty lost at this point. Any help would be appreciated!

Edit 1 (Entity relationship configuration)

 modelBuilder.Entity<Student>(entity =>           {                           
                entity.HasOne(d => d.Teacher)
                    .WithMany(p => p.Students)
                    .HasForeignKey(d => d.TeacherId)
                    .OnDelete(DeleteBehavior.ClientSetNull)
                    .HasConstraintName("FK_Students_Teachers");
            });

This is my configuration

Answer 1

Try adding the reverse config to your parents entity:

 modelBuilder.Entity<Teacher>(entity =>           
            {                           
                entity.HasMany(d => d.Students)
                    .WithOne(p => p.Teacher)
                    .HasForeignKey(d => d.TeacherId);
            });