在db中插入列表作为外键值

Question

I have that db

And Student can have multiple Subjects. So I'm storing them like that:

StudentId: 1, Name: Math; StudentId: 1, Name: Geography

etc..

I have subjects in Viewbag:

ViewBag.Subjects = new SelectList(new[] { "Math", "Phisic", "Chemic", "English" }
                .Select(x => new { value = x, text = x }),
                "Value", "Text");

And user can select multiple items at once. When I'll register new student It must insert that student's subjects as well. For them I'm that:

        [HttpPost]
        [ValidateAntiForgeryToken]
        public async Task<ActionResult> Register([Bind(Include = "Id,Name,Surname,BirthDate,Gender,Subject")] Students student)
        {
            if (!ModelState.IsValid)
            {
                SubjectsList();
                return View();
            }
            Student _student = new Student();
            _student.Name = student.Name;
            _student.Surname = student.Surname;
            _student.Gender = student.Gender;
            _student.BirthDate = student.BirthDate;

            Subject _subject = new Subject();
            if (!student.Subject.Any())
            {
                foreach (var item in student.Subject)
                {
                   _db.Subjects.Add(item);//Here it throws an exception Wrong overload
                }
            }


            _db.Students.Add(_student);
            await _db.SaveChangesAsync();

            return RedirectToAction("Index");
        }

And my Students class is here:

public class Students
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Surname { get; set; }
    public string Gender { get; set; }
    public DateTime? BirthDate { get; set; }
    public List<string> Subject { get; set; }
}

My question is how can I add that subject values in my Subject table

Answer 1

Your classes should look like this:

public class Subject
{
    public int Id {get;set;}
    public string Name {get;set;}
    public virtual List<Student> Students {get;set;}
}

public class Student
{
    public int Id {get;set;}
    public string Name {get;set;}
    public virtual List<Subject> Subjects {get;set;}
}

You can then add subjects to students like this:

var subject = new Subject 
{ 
    Name = "Physics" 
};
_db.Subjects.Add(subject);

var student = new Student 
{ 
    Name = "venerik", 
    Subjects = new List<Subject> { subject }
};
_db.Students.Add(student);
_db.SaveChanges();

In this set up EF will create a table for both Student and Subject and an intersection table called SubjectStudent (as @Clockwork-Muse already suggested).

Answer 2

I am going to refrain on the potential for improving your data model to better reflect reality for now (as there is already some great discussion on this in the comments) , so given the constraints of what your actual data model looks like, here's my answer:

The way EF (and plain old Linq-To-SQL) are designed handles foreign key relationships at the ORM level, so that you do not have to.

The ORM entities generated by EF where foreign keys are involved create one-to-many relationships as List objects, and these are called navigation properties . This article sheds some light on how this works.

Thus, you do not need to add the individual subjects to the Subject table explicitly;

Since the Student and Subject tables have a foreign key relationship, having the List<Subject> on the Student objects is sufficient and EF will handle the inserts for you.

---

A note on the exception you're getting:

Your current code has a list of strings for subjects on the student class:

public List<string> Subject { get; set; }

Then you are trying to insert the strings into the _db.Subjects ( DbSet<Subject> ) instance, which is why you're getting the exception.

---

Solution:

• Make a Subjects class that mimics your EF Subject table (since it appears you're creating custom classes to mimic the ORM entities, which is a practice I personally like since it decouples the ORM from the final consumer of the data such as your front end)

• Change the Students class to have List<Subjects> instead of List<string> for the Subjects property.

Then your code to insert the records would look something roughly like this:

Student _student = new Student();
_student.Name = student.Name;
_student.Surname = student.Surname;
_student.Gender = student.Gender;          
_student.BirthDate = student.BirthDate;

// I removed the ! here because it seems pointless to check that you have no elements, and then try to add them....
if (student.Subject.Any())
{
    foreach (var item in student.Subjects)
    {
         // this is the ORM subject now, the names seem ambiguous 
         // but the important thing is you have are filling the List<Subject>
         // of the Student ORM entity with Subject ORM entities,
         // which will cause the ORM to handle the foreign key relationship
         // and insert items into the Subjects table

         _student.Subjects.Add(new Subject() { 
             Name = item.Name
         });

         // Note: the ORM will handle the StudentId field, and I assume
         // the Id field is an SQL Identity and will be auto-generated;
         // If not then, aside from, 'why not?', you'd populate it manually
    }
}

Then you still just do _db.Students.Add(_student); and it will work.