如果未引用任何项目，则删除SQL

问题描述

我正在图书馆管理网站上工作。我的数据库中有一个LibraryItem和Category表。

我想做的是，如果任何库项目中引用了任何类别，我希望用户不能删除它们。

因此，假设用库项目引用了名为“ DVD”的类别，并假设用户要删除“ DVD”类别。在这种情况下，我想告诉用户：

“无法删除该类别，因为它在一个项目中被引用，如果要删除DVD类别，请先删除该项目”

类别控制器如下：

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.Entity;
using System.Linq;
using System.Net;
using System.Web;
using System.Web.Mvc;
using LibraryProject;

namespace LibraryProject.Controllers
{
    public class CategoriesController : Controller
    {
        private LibraryDbEntities db = new LibraryDbEntities();

        // GET: Categories
        public ActionResult Index()
        {
            return View(db.Category.ToList());
        }

        // GET: Categories/Details/5
        public ActionResult Details(int? id)
        {
            if (id == null)
            {
                return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
            }
            Category category = db.Category.Find(id);
            if (category == null)
            {
                return HttpNotFound();
            }
            return View(category);
        }

        // GET: Categories/Create
        public ActionResult Create()
        {
            return View();
        }

        // POST: Categories/Create
        // To protect from overposting attacks,enable the specific properties you want to bind to,for 
        // more details see https://go.microsoft.com/fwlink/?LinkId=317598.
        [HttpPost]
        [ValidateAntiForgeryToken]
        public ActionResult Create([Bind(Include = "Id,CategoryName")] Category category)
        {

            if (ModelState.IsValid)
            {
                // Check if the cat name exists or not
                var cat = db.Category.FirstOrDefault(x => x.CategoryName == category.CategoryName);
                if (cat == null)
                {
                    db.Category.Add(category);
                    db.SaveChanges();
                    return RedirectToAction("Index");
                }
                else
                {
                    // Add here the logic required to inform your user about the problem
                    ViewBag.ErrorMessage = "This Category name already exists,choose another one!";

                }
            }
            return View(category);
        }

        // GET: Categories/Edit/5
        public ActionResult Edit(int? id)
        {
            if (id == null)
            {
                return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
            }
            Category category = db.Category.Find(id);
            if (category == null)
            {
                return HttpNotFound();
            }
            return View(category);
        }

        // POST: Categories/Edit/5
        // To protect from overposting attacks,for 
        // more details see https://go.microsoft.com/fwlink/?LinkId=317598.
        [HttpPost]
        [ValidateAntiForgeryToken]
        public ActionResult Edit([Bind(Include = "Id,CategoryName")] Category category)
        {
            if (ModelState.IsValid)
            {
                db.Entry(category).State = EntityState.Modified;
                db.SaveChanges();
                return RedirectToAction("Index");
            }
            return View(category);
        }

        // GET: Categories/Delete/5
        public ActionResult Delete(int? id)
        {
            if (id == null)
            {
                return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
            }
            Category category = db.Category.Find(id);
            if (category == null)
            {
                return HttpNotFound();
            }
            return View(category);
        }

        // POST: Categories/Delete/5
        [HttpPost,ActionName("Delete")]
        [ValidateAntiForgeryToken]
        public ActionResult DeleteConfirmed(int id)
        {
            Category category = db.Category.Find(id);
            db.Category.Remove(category);
            db.SaveChanges();
            return RedirectToAction("Index");
        }

        protected override void dispose(bool disposing)
        {
            if (disposing)
            {
                db.dispose();
            }
            base.dispose(disposing);
        }
    }
}

我知道我必须在“删除”中编写一行代码，但是我只是不知道如何...所有帮助将不胜感激。

解决方法

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