如何 add/update 与 EF 核心中实体的一端建立多对多关系

Question

我们已经访问了 EF Core 多对多更新中的很多链接，但无法找到我们问题的具体答案并澄清我们的理解。

场景：我们希望 add/update 一个实体及其关联的多对多关系，例如 (dbset.Add() 或 dbset.Update())

我们正在尝试以下操作，但只能 add/update 父实体而不是多对多关系列表。你能帮我们知道我们哪里错了吗？可以做什么？

当前模型结构：


        public class Teacher
    {
        [DatabaseGenerated(DatabaseGeneratedOption.Identity), Required]
        public long Id { get; set; }
        public string Name { get; set; }
        public List<TeacherDuty> TeacherDuties { get; set; }
    }
    public class Duty
    {
        [DatabaseGenerated(DatabaseGeneratedOption.Identity), Required]
        public long Id { get; set; }
        public string Description { get; set; }
        public List<TeacherDuty> TeacherDuties { get; set; }
    }
    public class TeacherDuty
    {
        public long TeacherId { get; set; }
        public Teacher Teacher { get; set; }

        public long DutyId { get; set; }
        public Duty Duty { get; set; }
    }

我们正在尝试 add/update 使用以下方法：

public async Task<Teacher> AddTeacher(Teacher pTeacher)
{
    try
    {
        return await _teacher.AddAsync(pTeacher);
    }
    catch (Exception ex) { throw ex; }
}

public async Task<Teacher> UpdateTeacher(Teacher pTeacher)
{
    try
    {
        return await _teacher.Update(pTeacher);
    }
    catch (Exception ex) { throw ex; }
}

如果可能，请指出我们对概念和解决方案的误解。谢谢

Answer 1

我创建了一个添加和编辑教师的演示。（_context 是数据库上下文）

添加老师：

[HttpPost]
[ValidateAntiForgeryToken]
public async Task<IActionResult> Create(Teacher teacher)
    {
        //get your desired dutyId with your own logic
        var SelectedDutyIds = new int[] { 1 };
        var teacherDuties = new List<TeacherDuty>();

        if (ModelState.IsValid)
        {
            _context.Add(teacher);
            await _context.SaveChangesAsync();


            foreach (var id in SelectedDutyIds)
            {
                var item = new TeacherDuty()
                {
                    TeacherId = teacher.Id,
                    DutyId = id,
                };
                teacherDuties.Add(item);
            }
            _context.AddRange(teacherDuties);
            await _context.SaveChangesAsync();
            return RedirectToAction(nameof(Index));
        }
        return View(teacher);
    }

编辑老师：先删除所有现有的TeacherDuties老师，然后再添加新的。

 [HttpPost]
 [ValidateAntiForgeryToken]
 public async Task<IActionResult> Edit(long id, Teacher teacher)
    {
        if (id != teacher.Id)
        {
            return NotFound();
        }

        if (ModelState.IsValid)
        {
            try
            {
               //your new dutyIds
                var newSelectedDutyIds = new int[] { 3 };
                var teacherDuties = new List<TeacherDuty>();


                var tdList = await _context.TeacherDuties.Where(td => td.TeacherId == teacher.Id).ToListAsync() ;
                _context.RemoveRange(tdList);

                foreach (var newid in newSelectedDutyIds)
                {
                    var item = new TeacherDuty()
                    {
                        TeacherId = teacher.Id,
                        DutyId = newid,
                    };
                    teacherDuties.Add(item);
                }
                _context.AddRange(teacherDuties);
                _context.Update(teacher);

                await _context.SaveChangesAsync();
            }
            catch (DbUpdateConcurrencyException)
            {
                if (!TeacherExists(teacher.Id))
                {
                    return NotFound();
                }
                else
                {
                    throw;
                }
            }
            return RedirectToAction(nameof(Index));
        }
        return View(teacher);
    }

参考

如何 add/update 与 EF 核心中实体的一端建立多对多关系

How to add/update many to many relations with one end of entity in EF core

c#

many-to-many

crud

asp.net-core-webapi

ef-core-2.2