Question

I’m using ASP.NET MVC4 EF CodeFirst.

Need help to write LINQ (to entities) code in Index action to get collection of Courses which are attended by selected student. The relationship is many to many with join table with payload.

//StudentController
//-----------------------

public ActionResult Index(int? id)
{
    var viewModel = new StudentIndexViewModel();
    viewModel.Students = db.Students;

    if (id != null)
    {
        ViewBag.StudentId = id.Value;
        // *************PROBLEM IN LINE DOWN. HOW TO MAKE COURSES COLLECTION? 
        viewModel.Courses = db.Courses
            .Include(i => i.StudentsToCourses.Where(t => t.ObjStudent.FkStudentId == id.Value));
    }


    return View(viewModel);
}

The error I got is:

The Include path expression must refer to a navigation property defined on the type. Use dotted paths for reference navigation properties and the Select operator for collection navigation properties.

I have modeles (the third one is for join table with payload):

//MODEL CLASSES
//-------------

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

    public virtual ICollection<StudentToCourse> StudentsToCourses { get; set; }
}

public class Course
{
    public int CourseId { get; set; }
    public string Title { get; set; }

    public virtual ICollection<StudentToCourse> StudentsToCourses { get; set; }
}

public class StudentToCourse
{
    public int StudentToCourseId { get; set; }
    public int FkStudentId { get; set; }
    public int FkCourseId { get; set; }
    public string Classroom { get; set; }

    public virtual Student ObjStudent { get; set; }
    public virtual Course ObjCourse { get; set; }
}

Then, here is modelview I need to pass to view

//VIEWMODEL CLASS
//---------------

public class StudentIndexViewModel
{
    public IEnumerable<Student> Students { get; set; }
    public IEnumerable<Course> Courses { get; set; }
    public IEnumerable<StudentToCourse> StudentsToCourses { get; set; }
}

Answer 1

EF does not support conditional include’s. You’ll need to include all or nothing (ie no Whereinside the Include)

If you need to get the data for just certain relations, you can select it into an anonymous type, something like (the obviously untested);

var intermediary = (from course in db.Courses
                    from stc in course.StudentsToCourses
                    where stc.ObjStudent.FkStudentId == id.Value
                    select new {item, stc}).AsEnumerable();

Obviously, this will require some code changes, since it’s no longer a straight forward Course with a StudentsToCourses collection.