如何從ASP.NET Core中的父類實例檢索嵌套類實例的數據？

Question

我試圖在學校模擬一個班級，結果卻是這樣的：

public class Class
{
    public int ID { get; set; }
    public int Grade { get; set; }
    public Teacher ClassTeacher { get; set; }
}

這是教師班：

public class Teacher
{
    public int ID { get; set; }

    [Display(Name = "First Name")]
    public string FirstName { get; set; }

    [Display(Name = "Last Name")]
    public string LastName { get; set; }

    [DataType(DataType.Date)]
    public DateTime Birthday { get; set; }  
}

當我使用腳手架，遷移和更新數據庫時，這是為我構建的實體框架結構：

dbo.Class:
ID: int
ClassTeacherID: int
Grade: int

dbo.Teacher:
ID: int
Birthday: datetime2(7)
FirstName: nvarchar(MAX)
LastName: nvarchar(MAX)

我想在Views\\Classes\\Details.cshtml顯示Teacher的FirstName ，但是即使在數據庫中創建了Teacher實例並將ClassTeacherID設置為新創建的Teacher ID之后， Model.ClassTeacher還是為null 。

尋找您的幫助。

編輯

ClassesController.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Rendering;
using Microsoft.EntityFrameworkCore;

namespace WebApplication1.Models
{
    public class ClassesController : Controller
    {
        private readonly WebApplication1Context _context;

        public ClassesController(WebApplication1Context context)
        {
            _context = context;    
        }

        // GET: Classes
        public async Task<IActionResult> Index()
        {
            return View(await _context.Class.ToListAsync());
        }

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

            var @class = await _context.Class
                .SingleOrDefaultAsync(m => m.ID == id);
            if (@class == null)
            {
                return NotFound();
            }

            return View(@class);
        }

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

        // POST: Classes/Create
        // To protect from overposting attacks, please 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,Grade")] Class @class)
        {
            if (ModelState.IsValid)
            {
                _context.Add(@class);
                await _context.SaveChangesAsync();
                return RedirectToAction("Index");
            }
            return View(@class);
        }

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

            var @class = await _context.Class.SingleOrDefaultAsync(m => m.ID == id);
            if (@class == null)
            {
                return NotFound();
            }
            return View(@class);
        }

        // POST: Classes/Edit/5
        // To protect from overposting attacks, please 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,Grade")] Class @class)
        {
            if (id != @class.ID)
            {
                return NotFound();
            }

            if (ModelState.IsValid)
            {
                try
                {
                    _context.Update(@class);
                    await _context.SaveChangesAsync();
                }
                catch (DbUpdateConcurrencyException)
                {
                    if (!ClassExists(@class.ID))
                    {
                        return NotFound();
                    }
                    else
                    {
                        throw;
                    }
                }
                return RedirectToAction("Index");
            }
            return View(@class);
        }

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

            var @class = await _context.Class
                .SingleOrDefaultAsync(m => m.ID == id);
            if (@class == null)
            {
                return NotFound();
            }

            return View(@class);
        }

        // POST: Classes/Delete/5
        [HttpPost, ActionName("Delete")]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> DeleteConfirmed(int id)
        {
            var @class = await _context.Class.SingleOrDefaultAsync(m => m.ID == id);
            _context.Class.Remove(@class);
            await _context.SaveChangesAsync();
            return RedirectToAction("Index");
        }

        private bool ClassExists(int id)
        {
            return _context.Class.Any(e => e.ID == id);
        }
    }
}

TeachersController.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Rendering;
using Microsoft.EntityFrameworkCore;

namespace WebApplication1.Models
{
    public class TeachersController : Controller
    {
        private readonly WebApplication1Context _context;

        public TeachersController(WebApplication1Context context)
        {
            _context = context;    
        }

        // GET: Teachers
        public async Task<IActionResult> Index()
        {
            return View(await _context.Teacher.ToListAsync());
        }

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

            var teacher = await _context.Teacher
                .SingleOrDefaultAsync(m => m.ID == id);
            if (teacher == null)
            {
                return NotFound();
            }

            return View(teacher);
        }

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

        // POST: Teachers/Create
        // To protect from overposting attacks, please 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,FirstName,LastName,Birthday")] Teacher teacher)
        {
            if (ModelState.IsValid)
            {
                _context.Add(teacher);
                await _context.SaveChangesAsync();
                return RedirectToAction("Index");
            }
            return View(teacher);
        }

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

            var teacher = await _context.Teacher.SingleOrDefaultAsync(m => m.ID == id);
            if (teacher == null)
            {
                return NotFound();
            }
            return View(teacher);
        }

        // POST: Teachers/Edit/5
        // To protect from overposting attacks, please 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,FirstName,LastName,Birthday")] Teacher teacher)
        {
            if (id != teacher.ID)
            {
                return NotFound();
            }

            if (ModelState.IsValid)
            {
                try
                {
                    _context.Update(teacher);
                    await _context.SaveChangesAsync();
                }
                catch (DbUpdateConcurrencyException)
                {
                    if (!TeacherExists(teacher.ID))
                    {
                        return NotFound();
                    }
                    else
                    {
                        throw;
                    }
                }
                return RedirectToAction("Index");
            }
            return View(teacher);
        }

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

            var teacher = await _context.Teacher
                .SingleOrDefaultAsync(m => m.ID == id);
            if (teacher == null)
            {
                return NotFound();
            }

            return View(teacher);
        }

        // POST: Teachers/Delete/5
        [HttpPost, ActionName("Delete")]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> DeleteConfirmed(int id)
        {
            var teacher = await _context.Teacher.SingleOrDefaultAsync(m => m.ID == id);
            _context.Teacher.Remove(teacher);
            await _context.SaveChangesAsync();
            return RedirectToAction("Index");
        }

        private bool TeacherExists(int id)
        {
            return _context.Teacher.Any(e => e.ID == id);
        }
    }
}

Answer 1

我知道問題已經寫了一段時間了，但是您應該嘗試包括所需的對象引用。

var @class = await _context.Class.Include("Teacher")
            .SingleOrDefaultAsync(m => m.ID == id);

Include將獲取依賴對象，並將其放回您的Class模型中。 如果您有不同的引用對象，則可以鏈接Include指令。

注意：您必須添加：

using Microsoft.EntityFrameworkCore;

在您的代碼之上以允許“包含”起作用。