How to insert a record into a table with a foreign key using Entity Framework in ASP.NET MVC

Question

I'm new to Entity Framework code-first. This is my learning in ASP.NET MVC, using code-first for database creation.

I have two classes:

public class Student
{
    public int StudentId { get; set; }
    public string Name { get; set; }
    public int Standard { get; set; }            
    public int SubjectId { get; set; }    

    [ForeignKey("SubjectId")]
    public ICollection<Subject> Subjects { get; set; }
}

public class Subject
{
    [Key]
    public int SubjectId{ get; set; }
    public string SubjectName { get; set; }
}

I'm trying to insert a Student record into the Student table, which has a foreign key SubjectId referencing the Subject table.

I'm trying it out in two possible ways:

First approach

using(var cxt = new SchoolContext())
{
    Subject sub = new Subject() { SubjectId = 202, SubjectName ="Geology" };
    Student stu = new Student() { Name = "Riya", SubjectId = 202 };
    cxt.Subjects.Add(sub);
    cxt.Students.Add(stu);           

    cxt.SaveChanges();
}

Here I created a new Subject instance, which has SubjectId=202. Now when I create the Student object and assign value 202 to SubjectId, there is an Insert statement conflict. Though there is a Subject with SubjectId = 202, then why is there an insert conflict? And when I debug, I see that the navigation property Subjects is null here. I don't understand the point here.

Second approach:

using( var cxt=new SchoolContext())
{
    Student stu = new Student() { Name = "Riya" };
    Subject sub = new Subject() { SubjectId = 202, SubjectName = "Geology" };
    stu.Subjects.Add(sub);
    cxt.Students.Add(stu);               

    cxt.SaveChanges();
}

But I get an a null reference exception

Object Reference not set to instance of an Object

Why is the stu.Subjects null here?

So my questions here are:

What does the SubjectId in Student class mean? I.e. what does its value pertain to? Can we explicitly set it, if yes, will it refer to primary key of Subject table? If no, is it specified only for EF code conventions purpose?
Similarly: what does navigation property role? Why is it null and when it will not be null?

My basic understanding of navigation property is that it is used to make EF determine the relationship between the two entities.

Can anyone please clarify a bit with examples which would be greatly appreciated.

There should be no SubjectId in your Student class. You are probably looking for many-to-many: a student can have many subjects, a subject can have many students. — Steve Greene
– Steve Greene, Commented Aug 18, 2017 at 14:39
In Student, make sure to mark your Subjects collection as virtual. — Tipx
– Tipx, Commented Aug 18, 2017 at 14:40
@Tipx - Virtual is only needed if lazy loading is desired. That's not the issue. — Steve Greene
– Steve Greene, Commented Aug 18, 2017 at 14:45
First you need to decide what kind of relation you need, second you need to initialize the foreign key object to be able to insert via a foreign key — Lucian Bumb
– Lucian Bumb, Commented Aug 18, 2017 at 14:52

marc_s · Accepted Answer · 2017-08-18 16:00:08Z

17

You're basically creating a new Student, and a new Subject, in both your approaches. But from what I understand, what you're really trying to do, is create a new Student, and assign an existing Subject (with SubjectId = 202) to it - right??

The SubjectId in your Student class makes absolutely no sense in this setup - since you have a 1:n relationship between Student and Subject. You need to use that ICollection<Subject> to handle the 0:n subjects this student is enrolled in.

For that - use this code:

using(var ctx = new SchoolContext())
{
    // create the *NEW* Student
    Student stu = new Student() { Name = "Riya" };

    // get existing subject with Id=202
    Subject sub = ctx.Subjects.FirstOrDefault(s => s.SubjectId == 202);

    // Add this existing subject to the new student's "Subjects" collection
    stu.Subjects.Add(sub);

    // Add the new student to the context, and save it all.
    ctx.Students.Add(stu);           

    ctx.SaveChanges();
}

That will do - a new student will be inserted into your database table, and the 1:n relationship between student and his subjects will be established.

edited Aug 18, 2017 at 16:00

answered Aug 18, 2017 at 15:53

marc_s

760k186 gold badges1.4k silver badges1.5k bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Raida Adn Over a year ago

Can't I do like this, Create a new Subject, call SaveChanges,next Create Student retreive Subject and add this subject to the new Student

Collectives™ on Stack Overflow

How to insert a record into a table with a foreign key using Entity Framework in ASP.NET MVC

1 Answer 1

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related