Violation of PRIMARY KEY constraint 'PK_StudentCourses'. Cannot insert duplicate key

c# entity-framework-core

Question

I'm a beginner and trying to create a many to many relation using a post request to user

/ApplyCourse/?studentId=1003&courseId=3

but I get the following error:

SqlException: Violation of PRIMARY KEY constraint 'PK_StudentCourses'. Cannot insert duplicate key in object 'dbo.StudentCourses'. The duplicate key value is (1003, 3).

What am I doing wrong? What is a good approach to create many to many relations?

[Route("CourseApply")]
[HttpPost("{studentId}/{courseId}")]
public async Task<IActionResult> CourseApply(int studentId, int courseId)
{
        Student studentEntity = _context.Students.Find(studentId);
        Course courseEntity = _context.Courses.Find(courseId);

        StudentCourse enrollment = new StudentCourse();
        enrollment.Course = courseEntity;
        enrollment.Student = studentEntity;

        _context.StudentCourses.Add(enrollment);

        await _context.SaveChangesAsync();

        return CreatedAtAction("GetStudent", new { id = studentEntity.Id }, studentEntity);
}

public class StudentCourse
{
    public int StudentId { get; set; }
    public Student Student { get; set; }

    public int CourseId { get; set; }
    public Course Course { get; set; }
}

public class Course
{
    public int Id { get; set; }
    public string Name { get; set; }

    public IList<StudentCourse> Students { get; set; }
}

public class Student
{
    public int Id { get; set; }
    public string Name { get; set; }

    public IList<StudentCourse> Courses { get; set; }
}
1
0
8/12/2018 6:26:39 AM

Accepted Answer

Your primary key is composite: StudentId, CourseId. You already have combination of StudentId = 1003; CourseId = 3 in the database; so you can't insert another row with the same values.

You need to decide whether you want to allow such duplicates - then create a separate key column (call it Id or StudentCourseId and entity framework will automatically use it as primary key). Or if you disallow duplicate combinations of Student / Course then you need to handle such error (for example, by redirecting the user to update existing record, rather than inserting a new one)

Personally, I try to avoid composite keys... but I guess, it's a matter of preference

1
8/11/2018 9:00:40 PM


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