EF Core Eager 加载返回 null

Posted 2023-04-12

【中文标题】EF Core Eager 加载返回 null

【英文标题】：EF Core Eager loading returns null

【发布时间】：2021-01-31 22:17:28

【问题描述】：

我有一个实体患者和一个实体用户。它们之间是一对一的关系。

用户：

public class User
          
        public long Id  get; set; 

        public string Name  get; set; 

        public string Username  get; set;       
                ......
        [JsonIgnore]
        public   Patient Patient  get; set; 
   

患者：

public class Patient
    
        public long Id  get; set; 

        public long UserId  get; set; 

        public  User User  get; set; 
    

当我尝试从数据库中提取患者时，我得到了该用户的空引用。

DataContext:
 protected override void OnModelCreating(ModelBuilder modelBuilder)
        
            modelBuilder.Entity<Patient>()
                .HasOne(b => b.User)
                .WithOne(i => i.Patient)
                .HasForeignKey<Patient>(b => b.UserId);

                 ....
    

患者回购：

public IEnumerable<Patient> GetAll()
        
            var patients = _dataContext.Patients.Include(x=>x.User);

            return patients;
        

控制器方法：

  [HttpGet("patients")]
        public IActionResult GetAllPatient()
        
            return Ok(_patientRepository.GetAll());
        

迁移： 用户

migrationBuilder.CreateTable(
                name: "Users",
                columns: table => new
                
                    Id = table.Column<long>(nullable: false)
                        .Annotation("mysql:ValueGenerationStrategy", MySqlValueGenerationStrategy.IdentityColumn),
                    Name = table.Column<string>(nullable: true),
                    Username = table.Column<string>(nullable: true),
                    PasswordHash = table.Column<byte[]>(nullable: true),
                    PasswordSalt = table.Column<byte[]>(nullable: true),
                    BirthDate = table.Column<DateTime>(nullable: false),
                    GenderId = table.Column<int>(nullable: true),
                    UserRoleId = table.Column<long>(nullable: true)
                ,
                constraints: table =>
                
                    table.PrimaryKey("PK_Users", x => x.Id);
                    table.ForeignKey(
                        name: "FK_Users_Genders_GenderId",
                        column: x => x.GenderId,
                        principalTable: "Genders",
                        principalColumn: "Id",
                        onDelete: ReferentialAction.Restrict);
                    table.ForeignKey(
                        name: "FK_Users_UserRoles_UserRoleId",
                        column: x => x.UserRoleId,
                        principalTable: "UserRoles",
                        principalColumn: "Id",
                        onDelete: ReferentialAction.Restrict);
                );

患者

migrationBuilder.CreateTable(
                name: "Patients",
                columns: table => new
                
                    Id = table.Column<long>(nullable: false)
                        .Annotation("MySql:ValueGenerationStrategy", MySqlValueGenerationStrategy.IdentityColumn),
                    UserId = table.Column<long>(nullable: false),
                    DoctorId = table.Column<long>(nullable: true),
                    CaregiverId = table.Column<long>(nullable: true)
                ,
                constraints: table =>
                
                    table.PrimaryKey("PK_Patients", x => x.Id);
                    table.ForeignKey(
                        name: "FK_Patients_Caregivers_CaregiverId",
                        column: x => x.CaregiverId,
                        principalTable: "Caregivers",
                        principalColumn: "Id",
                        onDelete: ReferentialAction.Restrict);
                    table.ForeignKey(
                        name: "FK_Patients_Doctors_DoctorId",
                        column: x => x.DoctorId,
                        principalTable: "Doctors",
                        principalColumn: "Id",
                        onDelete: ReferentialAction.Restrict);
                    table.ForeignKey(
                        name: "FK_Patients_Users_UserId",
                        column: x => x.UserId,
                        principalTable: "Users",
                        principalColumn: "Id",
                        onDelete: ReferentialAction.Cascade);
                );

我在 Postman 上收到的信息：

 
        "id": 1,
        "userId": 3,
        "user": null

调试时

我注意到一个奇怪的事情是 Mysql 将这种关系视为多对一

【问题讨论】：

评论不用于扩展讨论；这个对话是moved to chat。

【参考方案1】：

在创建两个实体之间的关系时应该设置DeleteBehavior：

protected override void OnModelCreating(ModelBuilder modelBuilder)
    
        modelBuilder.Entity<Patient>()
            .HasOne(b => b.User)
            .WithOne()
            .HasForeignKey<Patient>(b => b.UserId)
            .OnDelete(DeleteBehavior.Restrict);

【参考方案2】：

显然有一些库为方法 .Include() 提供功能：Microsoft.EntityFrameworkCore（好的）和System.Data.Entity（坏的）；强> 我选择了坏的，我没有语法错误或运行时错误，只是功能不好。

【参考方案3】：

好的，您需要在查询中使用.ToList()。

      var patients = _dataContext.Patients.Include(x=>x.User).ToList();

【讨论】：

我已经更改了它，我仍然收到 null。