ASP.NET CORE 3.1 中通过存储库模式的多对多关系

Question

I've made many to many relationship in ASP.NET Core and there are two tables Category and Subject This is Category Model

public class Category
{
    [Key]
    public int Id { get; set; }

    public string  Name { get; set; }

    public List<CategorySubject> CategorySubjects { get; set; } = new List<CategorySubject>();
   
}

This is subject model

 public class Subject
{
    [Key]
    public int Id { get; set; }

    public string Name { get; set; }

    public string Exam_Time { get; set; }

    public List<CategorySubject> CategorySubjects { get; set; }
}

This is CategorySubject Model

public class CategorySubject
{
    public  int CategoryId { get; set; }

    public int  SubjectId { get; set; }

    public Category Category { get; set; }

    public Subject Subject { get; set; }
}

This is part of DatabaseContext

public DbSet<CategorySubject> CategorySubjects { get; set; }
  protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<CategorySubject>().HasKey(pt => new { pt.CategoryId, pt.SubjectId });
        modelBuilder.Entity<CategorySubject>().HasOne(pt => pt.Category)
            .WithMany(pt => pt.CategorySubjects).HasForeignKey(p => p.CategoryId);

        modelBuilder.Entity<CategorySubject>().HasOne(pt => pt.Subject)
          .WithMany(pt => pt.CategorySubjects).HasForeignKey(p => p.SubjectId);
    }

I made one helper class by the name of Helper

 public class Helpers:Profile
{
    public Helpers()
    {
        CreateMap<Subject, SubjectViewModel>().ReverseMap();
        CreateMap<SubjectViewModel, Subject>();
        CreateMap<Category, CategoryViewModel>().ReverseMap();
    }
}

this is category service:

 public void Insert(Category category)
    {
        _context.Categories.Add(category);
    }

    public void Update(Category category)
    {
        _context.Categories.Update(category);
    }

This is CategoryController :

// GET: CategoryController/Create
    public IActionResult Create()
    {

        var subjectFromRepo = _categorySubject.Subject.GetAll();
        var selectList = new List<SelectListItem>();
        foreach (var item in subjectFromRepo)
        {
            selectList.Add(new SelectListItem(item.Name, item.Id.ToString()));
        }

        var vm = new CategoryViewModel()
        {
            Subjects = selectList
        };

        return View(vm);
    }

   
 // POST: CategoryController/Create
    [HttpPost]
    [ValidateAntiForgeryToken]
    public IActionResult Create(CategoryViewModel vm )
    {
        try
        {
            Category category = new Category()
            {
                Name = vm.Name
                
            };
            foreach(var item in vm.SelectedSubjects)
            {
                category.CategorySubjects.Add(new CategorySubject()
                {
                    SubjectId = Int32.Parse(item)
                }); 
            }
            _categorySubject.Category.Insert(category);
            _categorySubject.Save();
            return RedirectToAction(nameof(Index));


            
        }
        catch
        {
            return View();
        }
    }

    // GET: CategoryController/Edit/5
    public IActionResult Edit(int id)
    {
        var category = _categorySubject.Category.GetCategoryById(id);
        var subjects = _categorySubject.Subject.GetAll();
        var selectsubjects = category.CategorySubjects.Select(x => new Subject()
        {
            Id = x.Subject.Id,
            Name = x.Subject.Name
        });

        var selectlist = new List<SelectListItem>();
        subjects.ForEach(i => selectlist.Add(new SelectListItem(i.Name, i.Id.ToString(), 
            selectsubjects.Select(x => x.Id).Contains(i.Id))));

        var vm = new CategoryViewModel()
        {
            Id= category.Id,
            Name = category.Name,
            Subjects = selectlist
        };
        return View(vm);
    }

    // POST: CategoryController/Edit/5
    [HttpPost]
    [ValidateAntiForgeryToken]
    public IActionResult Edit(CategoryViewModel vm)
    {
        try
        {
            var category = _categorySubject.Category.GetCategoryById(vm.Id);
            category.Name = vm.Name;
            var selectedSubjects = vm.SelectedSubjects;
            var existingSubjects = category.CategorySubjects.Select(x => x.SubjectId.ToString()).ToList();
            var toAdd = selectedSubjects.Except(existingSubjects).ToList();
            var toRemove = existingSubjects.Except(selectedSubjects).ToList();
            var CategorySubjects = category.CategorySubjects.Where(x => !toRemove.Contains(x.SubjectId.ToString())).ToList();
            foreach (var item in toAdd)
            {
                category.CategorySubjects.Add(new CategorySubject()
                {
                    SubjectId = Int32.Parse(item),
                    CategoryId = Int32.Parse(item)
                });
            }
            _categorySubject.Save();
            return RedirectToAction("Index", "Category");
        }
        catch
        {
            return View();
        }
    }

This is Create.cshtml of Category :

<div class="style-form">
    <h2 class="text-center mt-3 mb-lg-3">Create New Category</h2>
    <form asp-action="Create">
        <div asp-validation-summary="ModelOnly" class="text-danger"></div>
        <div class="row">
            <div class="col-md-3 col-lg-3 col-sm-3"></div>
            <div class="col-md-6 col-lg-6 col-sm-6">
                <div class="input-group">
                    <div class="input-group-prepend">
                        <span class="input-group-text"><strong>Name:</strong></span>
                    </div>
                    <input asp-for="Name" class="form-control input-hover" placeholder="Enter Name.." />
                    <span asp-validation-for="Name" class="text-danger"></span>
                </div><br />
                <div class="input-group">
                    <div class="input-group-prepend">
                        <span class="input-group-text"><strong>Subject:</strong></span>
                    </div>
                    <select asp-for="SubjectId" class="form-control input-hover" asp-items="@Model.Subjects ">
                        <option value="">Please choose a Subject...</option>
                    </select>
                    <span asp-validation-for="SubjectId" class="text-danger"></span>
                </div><br />
            </div>
        </div>
        <div class="row">
            <div class="col-md-3 col-lg-3 col-sm-3"></div>
            <div class="col-md-6 col-lg-6 col-sm-6">
                <div class="form-group">
                    <button type="button" class="btn btn-backToList">
                        <a asp-action="Index">Back to List</a>
                    </button>
                    <button type="submit" class="btn btn-create">Create</button>
                </div>
                </div>
            </div>
         </form>

There when I click on the create new category button I can get data of subject form drop down list, but when I want to submit it I face this error:

NullReferenceException: Object reference not set to an instance of an object. AspNetCore.Views_Category_Create.b__20_0() in Create.cshtml, line 27

<select asp-for="SubjectId" class="form-control input-hover" asp-items="@Model.Subjects ">

Answer 1

I think there is an exception thrown in the Create (POST) method, it then goes to the catch, which returns a view without a model

    catch
    {
        return View();
    }

The next exception comes while rendering the page trying to bind to @Model.Subjects where Model is null.

Remove try/catch or handle the catch to find if there is any exception.