如何加载导航属性？

Question

I'm attempting to create a query using eager loading of navigational properties. Is there a way to load a property of a navigational property? I get a 'sequence contains no element' exception when I try the code below.

       var viewModel = new InstructorIndexData();
        viewModel.Instructors = db.Instructors.Include(i => i.OfficeAssignment)
            .Include(i=>i.Assignments) 
             .Include(i => i.Students)
             .Include (i=>i.Enrollments )
            .Include(i => i.Courses.Select(c => c.Department))
            .OrderBy(i => i.LastName);

        viewModel.Courses = viewModel.Instructors.Where(i => i.InstructorID == id).Single().Courses;
        viewModel.Enrollments = viewModel.Courses.Where(x => x.CourseID == courseID).Where( e=>e.Enrollments.First().InstructorFullName == "Kim Abercrombie").Single().Enrollments;

However when I use:

            viewModel.Enrollments = viewModel.Courses.Where(x => x.CourseID == courseID).Single().Enrollments;

it populates my viewModel.Enrollment property just fine. Is there a way to access this type of complex property in a linq query?

Answer 1

Is there a way to load a property of a navigational property?

Looks like you know how to eager load multiple levels already:

viewModel.Instructors = db.Instructors.Include(i => i.OfficeAssignment)
        .Include(i=>i.Assignments) 
         .Include(i => i.Students)
         .Include (i=>i.Enrollments )
        .Include(i => i.Courses.Select(c => c.Department))
        .OrderBy(i => i.LastName);

This is where you are probably getting the exception (or possibly your other use of .Single() ):

viewModel.Enrollments = viewModel.Courses
   .Where(x => x.CourseID == courseID)
   .Where( e=>e.Enrollments.First().InstructorFullName == "Kim Abercrombie")
   .Single().Enrollments;

From the docs for .Single()

Returns the only element of a sequence, and throws an exception if there is not exactly one element in the sequence.

Answer 2

Here is how I solved it. var selectTemp = viewModel.Courses.Where(x => x.CourseID == courseID).Single().Enrollments;

            var selectedTeacher = from t in selectTemp where t.InstructorFullName == teacher select t;


            viewModel.Enrollments = selectedTeacher;