物體框架：為什么我不能在這種多對多關系中清空集合？

課程類別：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace Library
{
    public class Course
    {
        public Course()
        {
            this.Students = new HashSet<Student>();
        }

        public string CourseId { get; set; }
        public string CourseName { get; set; }

        public string Department {get; set;}

        public virtual ICollection<Student> Students { get; set; }
    }
}

學生班

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Threading.Tasks;

namespace Library
{
    public class Student
    {
        public Student() 
        {
            this.Courses = new HashSet<Course>();
        }

        public string StudentId { get; set; }

        [Required]
        public string StudentName { get; set; }
        public int Age { get; set; }

        public virtual ICollection<Course> Courses { get; set; }
    }
}

我的方法（天藍色函式）

[FunctionName("UpdateCourse")]
public async Task<IActionResult> UpdateCourse(
[HttpTrigger(AuthorizationLevel.Anonymous, "put", Route = "courses/{CourseId}")]
HttpRequest req,
ILogger log, string CourseId) {

    var course = await _context.Courses.FindAsync(CourseId);

// course.Students.Clear();

    foreach (var item in course.Students)
    {
        course.Students.Remove(item);
    }


    _context.Update(course);

    await _context.SaveChangesAsync();

    return new OkObjectResult(removeSelfReference(course));
}

編輯1：

這是我的連接表的代碼優先遷移檔案的一部分CourseStudent

20220415034607_M1.cs：

migrationBuilder.CreateTable(
    name: "CourseStudent",
    columns: table => new
    {
        CoursesCourseId = table.Column<string>(type: "nvarchar(450)", nullable: false),
        StudentsStudentId = table.Column<string>(type: "nvarchar(450)", nullable: false)
    },
    constraints: table =>
    {
        table.PrimaryKey("PK_CourseStudent", x => new { x.CoursesCourseId, x.StudentsStudentId });
        table.ForeignKey(
            name: "FK_CourseStudent_Courses_CoursesCourseId",
            column: x => x.CoursesCourseId,
            principalTable: "Courses",
            principalColumn: "CourseId",
            onDelete: ReferentialAction.Cascade);
        table.ForeignKey(
            name: "FK_CourseStudent_Students_StudentsStudentId",
            column: x => x.StudentsStudentId,
            principalTable: "Students",
            principalColumn: "StudentId",
            onDelete: ReferentialAction.Cascade);
    });

20220415034607_M1.Designer.cs：

modelBuilder.Entity("CourseStudent", b =>
    {
        b.HasOne("Library.Course", null)
            .WithMany()
            .HasForeignKey("CoursesCourseId")
            .OnDelete(DeleteBehavior.Cascade)
            .IsRequired();

        b.HasOne("Library.Student", null)
            .WithMany()
            .HasForeignKey("StudentsStudentId")
            .OnDelete(DeleteBehavior.Cascade)
            .IsRequired();
    });

uj5u.com熱心網友回復：

您是否確認您的課程延遲加載學生？如果延遲加載被禁用，那么 course.Students 將為空。在修改學生集合時，您應該預先加載它，無論：

var course = context.Courses
    .Include(x => x.Students)
    .Single(x => x.CourseId == courseId);

從那里你應該能夠使用：

course.Students.Clear(); 

因為 Course.Students 將是 EF 識別的代理，并且 Clear 方法會將所有條目標記為洗掉。對于多對多關系，您可能需要將課程與每個學生解除關聯：

var course = context.Courses
    .Include(x => x.Students)
    .Single(x => x.CourseId == courseId);

foreach(var student in course.Students)
    student.Courses.Remove(course);

course.Students.Clear();
context.SaveChanges();

以確保在任何學生可能被標記為已修改的情況下洗掉參考。

最后，對于被跟蹤的物體，只需呼叫SaveChanges()而不是呼叫Update。Update用于未跟蹤/分離的物體。

標籤：C＃ 。网 实体框架 林克 实体框架核心

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