How to stop creating extra records in joined tables when I create new record in join table in EF Core

I have 3 linked tables - Students, Courses and Enrollments. When I try create a new Enrollment with EF Core, the table is created with StudentId and CourseId foreign keys but also extra records in the Student and Courses tables which I do not want. I know this is a fairly common situation and have looked at dozens of answers to this question - but am clearly missing something basic.

Here's the definition of the 3 classes:

public class Course
{
    public int Id { get; set; }
    public string Name { get; set; } = string.Empty;
    public List<Enrollment>? Enrollments { get; set; }
}

public class Student
{
    public string Id { get; set; } = string.Empty;
    public string Name { get; set; } = string.Empty;
    public List<Enrollment>? Enrollments { get; set; }
}

public class Enrollment
{
    public int Id { get; set; }
    public string StudentId { get; set; } = string.Empty;
    public virtual Student Student { get; set; } = new Student();

    public int CourseId { get; set; }
    public virtual Course Course { get; set; } = new Course();

    public string Name { get; set; } = string.Empty;
}

I've created a little app at https://github.com/rswetnam/MultiJoinEF which demonstrates this problem. Any help on fixing this would be greatly appreciated.

Find the Create.cshtml.cs file , and change your code inside the OnPostAsync method like below, and you issue will be fixed.

    // To protect from overposting attacks, see https://aka.ms/RazorPagesCRUD
    public async Task<IActionResult> OnPostAsync()
    {
      if (!ModelState.IsValid || _context.Enrollments == null || Enrollment == null)
        {
            return Page();
        }
        Course existingCourse = await _context.Courses.FindAsync(Enrollment.CourseId);
        Enrollment.Course = existingCourse;
        Student existingStudent = await _context.Students.FindAsync(Enrollment.StudentId);
        Enrollment.Student = existingStudent;
        _context.Enrollments.Add(Enrollment);
        await _context.SaveChangesAsync();

        return RedirectToPage("./Index");
    }