InstructorsController.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using Microsoft.EntityFrameworkCore;
using MvcUniversity.Data;
using MvcUniversity.Models;

namespace MvcUniversity.Controllers
{
    public class InstructorsController : Controller
    {
        private readonly MvcUniversityContext _context;

        public InstructorsController(MvcUniversityContext context)
        {
            _context = context;
        }

        // GET: Instructors
        public async Task<IActionResult> Index()
        {
            var instructors = await _context.Instructors
                .Include(i => i.Courses)
                .AsNoTracking()
                .ToListAsync();

            return View(instructors);
        }

        // GET: Instructors/Details/5
        public async Task<IActionResult> Details(int? id)
        {
            if (id == null)
            {
                return NotFound();
            }

            var instructor = await _context.Instructors
               .Include(i => i.Courses)
               .AsNoTracking()
               .FirstOrDefaultAsync(m => m.Id == id);

            if (instructor == null)
            {
                return NotFound();
            }

            return View(instructor);
        }

        // GET: Instructors/Create
        public IActionResult Create()
        {
            return View();
        }

        // POST: Instructors/Create
        // To protect from overposting attacks, enable the specific properties you want to bind to.
        // For more details, see http://go.microsoft.com/fwlink/?LinkId=317598.
        [HttpPost]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> Create([Bind("Id,LastName,FirstName,HireDate")] Instructor instructor)
        {
            if (ModelState.IsValid)
            {
                _context.Add(instructor);
                await _context.SaveChangesAsync();
                return RedirectToAction(nameof(Index));
            }
            return View(instructor);
        }

        // GET: Instructors/Edit/5
        public async Task<IActionResult> Edit(int? id)
        {
            if (id == null)
            {
                return NotFound();
            }

            var instructor = await _context.Instructors
              .Include(i => i.Courses)
              .AsNoTracking()
              .FirstOrDefaultAsync(m => m.Id == id);

            if (instructor == null)
            {
                return NotFound();
            }
            PopulateAssignedCourseViewModel(instructor);
            return View(instructor);
        }

        // POST: Instructors/Edit/5
        // To protect from overposting attacks, enable the specific properties you want to bind to.
        // For more details, see http://go.microsoft.com/fwlink/?LinkId=317598.
        [HttpPost]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> Edit(int id, [Bind("Id,LastName,FirstName,HireDate")] Instructor instructor, string[] selectedCourses)
        {
            if (id != instructor.Id)
            {
                return NotFound();
            }

            if (ModelState.IsValid)
            {
                try
                {
                    _context.Update(instructor);
                    UpdateInstructorCourses(selectedCourses, instructor.Id);
                    await _context.SaveChangesAsync();
                }
                catch (DbUpdateConcurrencyException)
                {
                    if (!InstructorExists(instructor.Id))
                    {
                        return NotFound();
                    }
                    else
                    {
                        throw;
                    }
                }
                return RedirectToAction(nameof(Index));
            }
            return View(instructor);
        }

        // GET: Instructors/Delete/5
        public async Task<IActionResult> Delete(int? id)
        {
            if (id == null)
            {
                return NotFound();
            }

            var instructor = await _context.Instructors
                .FirstOrDefaultAsync(m => m.Id == id);
            if (instructor == null)
            {
                return NotFound();
            }

            return View(instructor);
        }

        // POST: Instructors/Delete/5
        [HttpPost, ActionName("Delete")]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> DeleteConfirmed(int id)
        {
            var instructor = await _context.Instructors.FindAsync(id);
            _context.Instructors.Remove(instructor);
            await _context.SaveChangesAsync();
            return RedirectToAction(nameof(Index));
        }

        private bool InstructorExists(int id)
        {
            return _context.Instructors.Any(e => e.Id == id);
        }

        // Add courses info to view data
        private void PopulateAssignedCourseViewModel(Instructor instructor)
        {
            var allCourses = _context.Courses;
            var instructorCourses = new HashSet<int>(
                instructor.Courses.Select(c => c.Id));
            var viewModel = new List<AssignedCourseViewModel>();
            foreach (var course in allCourses)
            {
                viewModel.Add(new AssignedCourseViewModel
                {
                    Id = course.Id,
                    Title = course.Title,
                    Assigned = instructorCourses.Contains(course.Id)
                });
            }
            ViewData["Courses"] = viewModel;
        }

        // Update instructor courses according to selections in Edit view
        public async void UpdateInstructorCourses(string[] selectedCourses, int instructorId)
        {
            // Load instructor to update from DB
            var instructor = await _context.Instructors
               .Include(i => i.Courses)
               .FirstOrDefaultAsync(m => m.Id == instructorId);

            if (selectedCourses == null)
            {
                instructor.Courses = new List<Course>();
                return;
            }

            var selectedCoursesHS = new HashSet<string>(selectedCourses);
            var instructorCourses = new HashSet<int>
                (instructor.Courses.Select(c => c.Id));
            foreach (var course in _context.Courses)
            {
                if (selectedCoursesHS.Contains(course.Id.ToString()))
                {
                    // Course has been selected in the UI
                    if (!instructorCourses.Contains(course.Id))
                    {
                        // Add relationship between instructor and course
                        instructor.Courses.Add(course);
                    }
                }
                else
                {
                    // Course has not been selected in the UI
                    if (instructorCourses.Contains(course.Id))
                    {
                        // Remove existing relationship between instructor and course
                        var courseToRemove = instructor.Courses.Single(
                                                        c => c.Id == course.Id);
                        instructor.Courses.Remove(courseToRemove);
                    }
                }
            }
        }
    }
}