C# How to remove reference between two entities using Entity Framework?

Question

I have models:

Student = (... List CourseList);
Course = (... List StudentList);

I added students to few different courses and using Entity Framework, I added everything to the database.

Please help me remove student from specific course using EF

https://stackoverflow.com/questions/14381689/delete-related-elements-in-1n-relation-and-not-only-inserts-null-in-the-foreign — Greg, Nov 02 '17 at 18:41
https://stackoverflow.com/questions/22858491/entity-framework-remove-object-with-foreign-key-preserving-parent — Greg, Nov 02 '17 at 18:42
https://stackoverflow.com/questions/17723626/entity-framework-remove-vs-deleteobject — Greg, Nov 02 '17 at 18:42

score 0 · Accepted Answer · answered Nov 02 '17 at 19:37

0

It greatly helps to show your models and code for assistance. I'll take a stab based on your pseudo code:

var course = context.Courses.Include(c => c.Students)
    .SingleOrDefault(c => c.CourseID == selectedCourseId);

if (course != null)
{
    var studentToRemove = course.Students.SingleOrDefault(s => c.StudentID == selectedStudentId);
    if (studentToRemove != null)
    {
        course.Students.Remove(studentToRemove);
        context.SaveChanges();
    }
}

answered Nov 02 '17 at 19:37

Steve Greene

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Thank you! :) And sorry for my bad explanation of my problem, i was on the phone and in a hurry, that will never happen again! :) – eruditusfaber Nov 02 '17 at 21:08

score 0 · Answer 2 · answered Nov 02 '17 at 20:58

It depends a bit if you have configured a one-to-many relationship or a many-to-many relationship

I think every Student can attend zero or more Courses, and every Course can be attended by zero or more Students: a many-to-many relation ship.

This is configured as follows:

class Student
{
   public int Id {get; set;}
   // a Student attends zero or more courses:
   public virtual ICollection<Course> Courses {get; set;}
}
class Course
{
   public int Id {get; set;}
   // a Course is attended by zero or more Students:
   public virtual ICollection<Student> Students{get; set;}
}
class MyDbContext : DbConter
{
    public DbSet<Student> Students {get; set;}
    public DbSet<Course> Courses {get; set;}
}

This is all. From this Entity Framework will know you want to design a many-to-many relationship. It will even create and fill the many-to-many junction table for you.

So now you've added some Students and Some Courses. Suppose you want to remove a Student.

using (var dbContext = new MyDbContext())
{
    Student studentToRemove = ... 
    // for example:
    Student studentToRemove = dbContext.Students
         .Where(student => student.Name == "John Doe")
         .FirstOrDefault();
    if (studentToRemove != null)
    {
        dbContext.Students.Remove(studentToRemove);
        dbConext.SaveChanges();
    }
}

That is all. Entity Framework will remove the Student from the Students table and automatically make sure that he can't be found on any of the Courses lists. You don't have to do some inner join. Entity Framework knows it is a many-to-many relationship and will do the correct inner join for you

C# How to remove reference between two entities using Entity Framework?

2 Answers2