ASP.NET Core MVC+Angular CRUD更新报错DbUpdateConcurrencyException求解

使用ASP.NET Core MVC结合Angular开发CRUD系统，新增、删除、查看功能均正常，但更新记录时失败，报错信息如下：

Microsoft.EntityFrameworkCore.DbUpdateConcurrencyException:
The database operation was expected to affect 1 row(s), but actually affected 0 row(s);

后端代码（StudentController.cs） #

using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using Microsoft.EntityFrameworkCore;
using ReactAspCrud.Models;

namespace ReactAspCrud.Controllers
{
    [Route("api/[controller]")]
    [ApiController]
    public class StudentController : ControllerBase
    {
        private readonly StudentDbContext _studentDbContext;

        public StudentController(StudentDbContext studentDbContext)
        {
            _studentDbContext = studentDbContext;
        }

        [HttpGet]
        [Route("GetStudent")]
        public async Task<IEnumerable<Student>> GetStudents()
        {
            return await _studentDbContext.Student.ToListAsync();
        }

        [HttpPost]
        [Route("AddStudent")]
        public async Task<Student> AddStudent(Student objStudent)
        {
            _studentDbContext.Student.Add(objStudent);
            await _studentDbContext.SaveChangesAsync();
            return objStudent;
        }

        [HttpPatch]
        [Route("UpdateStudent/{id}")]
        public async Task<Student> UpdateStudent(Student objStudent)
        {
            _studentDbContext.Entry(objStudent).CurrentValues.SetValues(objStudent);
            _studentDbContext.Entry(objStudent).State= EntityState.Modified;
            await _studentDbContext.SaveChangesAsync();
            return objStudent;
        }

        [HttpDelete]
        [Route("DeleteStudent/{id}")]
        public bool DeleteStudent(int id) 
        {
            bool a = false;
            var student = _studentDbContext.Student.Find(id);
            if (student != null)
            {
                a = true;
                _studentDbContext.Entry(student).State= EntityState.Deleted;
                _studentDbContext.SaveChanges();
            }
            else
            {
                a = false;
            }
            return a;
        }
    }
}

前端代码（Studentcrud.components.ts） #

setUpdate(data: any) 
{
  this.stname = data.stname;
  this.course = data.course;
  this.currentStudentID = data.id;
}

UpdateRecords()
{
  let bodyData = 
  {
    "stname" : this.stname,
    "course" : this.course,
  };
  
  this.http.patch("https://localhost:7195/api/Student/UpdateStudent"+ "/"+ this.currentStudentID,bodyData).subscribe((resultData: any)=>
  {
      console.log(resultData);
      alert("Student Registered Updateddd")
      this.getAllStudent();
  });
}

问题原因及修复方案 #

核心问题

1. 后端UpdateStudent方法定义了路由参数{id}，但方法未接收该参数，也没有通过主键从数据库查询目标实体，直接修改未被EF Core追踪的对象，导致无法定位数据库中的记录。
2. 前端请求的Body中未包含id字段，后端接收的Student对象主键为空，EF Core找不到要更新的行。

修复步骤

后端修改（推荐方案）

先通过路由参数id查询数据库中的实体，再更新指定字段，最后保存变更：

[HttpPatch]
[Route("UpdateStudent/{id}")]
public async Task<IActionResult> UpdateStudent(int id, Student objStudent)
{
    var existingStudent = await _studentDbContext.Student.FindAsync(id);
    if (existingStudent == null)
    {
        return NotFound("学生记录不存在");
    }
    // 更新需要修改的字段
    existingStudent.stname = objStudent.stname;
    existingStudent.course = objStudent.course;
    
    await _studentDbContext.SaveChangesAsync();
    return Ok(existingStudent);
}

前端可选优化

如果需要保持后端直接接收完整实体的方式，可在请求Body中加入id字段：

UpdateRecords()
{
  let bodyData = 
  {
    "id": this.currentStudentID,
    "stname" : this.stname,
    "course" : this.course,
  };
  
  this.http.patch(`https://localhost:7195/api/Student/UpdateStudent/${this.currentStudentID}`, bodyData).subscribe((resultData: any)=>
  {
      console.log(resultData);
      alert("学生记录更新成功")
      this.getAllStudent();
  });
}

内容的提问来源于stack exchange，提问作者abi jega