EF Core反向导航属性解决多对一关系

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发布时间：2019-06-19

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多对一是一种很常见的关系，例如：一个班级有一个学生集合属性，同时，班级有班长、语文课代表、数学课代表等单个学生属性，如果定义2个实体类，班级SchoolClass和学生Student，那么，班级SchoolClass类有多个学生Student类的导航属性，学生Student类有一个班级SchoolClass类的导航属性。此时就需要使用InverseProperty反向导航属性去指定通过哪个属性建立引用关系，否则数据库建不起来。

通过一个小DEMO做试验。

新建Asp.Net Core MVC网站项目，添加2个实体类如下所示

//班级    public class SchoolClass    {        //主键        public int ID { get; set; }        //班级名字        public string ClassTitle { get; set; }        //本班级的学生集合        public List
     
       Students { get; set; }        //班长        public Student ClassMonitor { get; set; }        //语文课代表        public Student Chinese { get; set; }        //数学课代表        public Student Mathematics { get; set; }    }    //学生    public class Student    {        //主键        public int ID { get; set; }        //姓名        public string Name { get; set; }        //学生所在的班级        public SchoolClass MyClass { get; set; }    }

然后通过右键菜单添加SchoolClass实体类的控制器，让系统自动创建数据库上下文代码

然后会收到一个错误。

Unable to determine the relationship represented by navigation property 'SchoolClass.Students' of type 'List<Student>'. Either manually configure the relationship, or ignore this property using the '[NotMapped]' attribute or by using 'EntityTypeBuilder.Ignore' in 'OnModelCreating'. StackTrace:

系统无法判断SchoolClass多个Student导航属性的关系，此时可以在Students属性上面添加反向导航属性[InverseProperty("MyClass")]，就可以完成自动化创建控制器了。

[InverseProperty("MyClass")]public List
     
       Students { get; set; }

然后在软件启动时创建一组测试数据。

public static void Main(string[] args)        {            //CreateWebHostBuilder(args).Build().Run();            IWebHost webHost = CreateWebHostBuilder(args).Build();            //系统初始化            AppInit(webHost.Services);            webHost.Run();        }        //系统初始化        private static void AppInit(IServiceProvider serviceProvider)        {            //初始化数据库            using (var scope = serviceProvider.CreateScope())            {                var context = scope.ServiceProvider.GetRequiredService
     
      ();                //确保创建数据库                context.Database.EnsureCreated();                if (context.SchoolClass.Any())                    return;                var schoolClass61 = new SchoolClass()                {                    ClassTitle = "六一班"                };                //先保存班级，否则报错                //Unable to save changes because a circular dependency was detected in the data to be saved: 'SchoolClass [Added] <- Students MyClass { 'MyClassID' } Student [Added] <- Chinese { 'ChineseID' } SchoolClass [Added]'.                context.Add(schoolClass61);                int rows = context.SaveChanges();                Console.WriteLine($"添加了班级{schoolClass61.ClassTitle}, 影响记录{rows}");                var student1 = new Student()                {                    Name = "张三",                };                var student2 = new Student()                {                    Name = "李四",                };                var student3 = new Student()                {                    Name = "王五",                };                var student4 = new Student()                {                    Name = "赵六",                };                schoolClass61.Students = new List
      
       ()                {                    student1,                    student2,                    student3,                    student4                };                //设置同学的职位                schoolClass61.ClassMonitor = student1;                schoolClass61.Chinese = student2;                schoolClass61.Mathematics = student3;                //保存到数据库                rows = context.SaveChanges();                Console.WriteLine($"添加了{schoolClass61.Students.Count}位同学, 影响记录{rows}");            }        }

然后修改控制器的Details方法，显示班级详细信息时Include加载全部学生集合Students，不需要再加载Chinese等各个课代表导航属性，因为已经加载了班上的全部学生，EF Core会自动处理这些Student类型的导航属性。

public async Task
     
       Details(int? id)        {            if (id == null)            {                return NotFound();            }            var schoolClass = await _context.SchoolClass                .Include(x => x.Students)                .FirstOrDefaultAsync(m => m.ID == id);            if (schoolClass == null)            {                return NotFound();            }            return View(schoolClass);        }

修改Details页面显示班级学生和各个职务的学生。


     
                  班上的同学        
     
        
     
                  @foreach (var student in Model.Students)            {                @student.Name
      
            }        
     
        
     
                  班长        
     
        
     
                  @Html.DisplayFor(model => model.ClassMonitor.Name)        
     
        
     
                  语文课代表        
     
        
     
                  @Html.DisplayFor(model => model.Chinese.Name)        
     
        
     
                  数学课代表        
     
        
     
                  @Html.DisplayFor(model => model.Mathematics.Name)

运行成功。

打开数据库连接，可以查看系统自动创建的外键引用，完全符合预期。

CREATE TABLE [dbo].[Student] (    [ID]        INT            IDENTITY (1, 1) NOT NULL,    [Name]      NVARCHAR (MAX) NULL,    [MyClassID] INT            NULL,    CONSTRAINT [PK_Student] PRIMARY KEY CLUSTERED ([ID] ASC),    CONSTRAINT [FK_Student_SchoolClass_MyClassID] FOREIGN KEY ([MyClassID]) REFERENCES [dbo].[SchoolClass] ([ID]));CREATE TABLE [dbo].[SchoolClass] (    [ID]             INT            IDENTITY (1, 1) NOT NULL,    [ClassTitle]     NVARCHAR (MAX) NULL,    [ClassMonitorID] INT            NULL,    [ChineseID]      INT            NULL,    [MathematicsID]  INT            NULL,    CONSTRAINT [PK_SchoolClass] PRIMARY KEY CLUSTERED ([ID] ASC),    CONSTRAINT [FK_SchoolClass_Student_ChineseID] FOREIGN KEY ([ChineseID]) REFERENCES [dbo].[Student] ([ID]),    CONSTRAINT [FK_SchoolClass_Student_ClassMonitorID] FOREIGN KEY ([ClassMonitorID]) REFERENCES [dbo].[Student] ([ID]),    CONSTRAINT [FK_SchoolClass_Student_MathematicsID] FOREIGN KEY ([MathematicsID]) REFERENCES [dbo].[Student] ([ID]));

继续试验，再增加一个老师实体类Teacher

//老师    public class Teacher    {        //主键        public int ID { get; set; }        //姓名        public string Name { get; set; }        //老师作为班主任管理的班级        public SchoolClass AdminClass { get; set; }    }

给班级SchoolClass增加班主任、语文老师、数学老师属性

//班主任        public Teacher HeadTeacher { get; set; }        //语文老师        public Teacher ChineseTeacher { get; set; }        //数学老师        public Teacher MathTeacher { get; set; }

修改Details方法，加载老师属性对象

var schoolClass = await _context.SchoolClass                .Include(x => x.Students)                .Include(x => x.HeadTeacher)                .Include(x => x.ChineseTeacher)                .Include(x => x.MathTeacher)                .FirstOrDefaultAsync(m => m.ID == id);

修改Details页面增加显示老师


     
                  班主任        
     
        
     
                  @Html.DisplayFor(model => model.HeadTeacher.Name)        
     
        
     
                  语文老师        
     
        
     
                  @Html.DisplayFor(model => model.ChineseTeacher.Name)        
     
        
     
                  数学老师        
     
        
     
                  @Html.DisplayFor(model => model.MathTeacher.Name)

补全StudentWebContext的数据表。

public class StudentWebContext : DbContext    {        public StudentWebContext (DbContextOptions
     
       options)            : base(options)        {        }        public DbSet
      
        SchoolClass { get; set; }        public DbSet
       
         Student { get; set; }        public DbSet
        
          Teacher { get; set; }    }

项目启动时增加老师的测试数据

//添加老师                var teacher1 = new Teacher()                {                    Name = "孔子"                };                var teacher2 = new Teacher()                {                    Name = "李白"                };                var teacher3 = new Teacher()                {                    Name = "祖冲之"                };                //设置老师的职位                schoolClass61.HeadTeacher = teacher1;                schoolClass61.ChineseTeacher = teacher2;                schoolClass61.MathTeacher = teacher3;                //保存到数据库                rows = context.SaveChanges();                Console.WriteLine($"添加了老师同学, 影响记录{rows}");

打开VS2017的SQL Server对象管理器，通过右键菜单粗暴删除SchoolClass、Student数据表，再次运行项目，再次收到类似的错误

Unable to determine the relationship represented by navigation property 'SchoolClass.HeadTeacher' of type 'Teacher'. Either manually configure the relationship, or ignore this property using the '[NotMapped]' attribute or by using 'EntityTypeBuilder.Ignore' in 'OnModelCreating'.

参照上述方法，给班级SchoolClass的班主任属性HeadTeacher增加反向导航属性[InverseProperty("AdminClass")]，这个问题就解决了。

//班主任        [InverseProperty("AdminClass")]        public Teacher HeadTeacher { get; set; }

再次运行，会收到新的错误

The child/dependent side could not be determined for the one-to-one relationship between 'Teacher.AdminClass' and 'SchoolClass.HeadTeacher'. To identify the child/dependent side of the relationship, configure the foreign key property. If these navigations should not be part of the same relationship configure them without specifying the inverse. See http://go.microsoft.com/fwlink/?LinkId=724062 for more details.

访问，自动跳转到，看介绍：

一对一

一对一关系两端具有引用导航属性。它们遵循相同的约定作为一个对多关系，但在外键属性，以确保只有一个依赖于与每个主体上引入了唯一索引。

不好理解，有点绕？看示例的代码，大约是把其中一个实体类的导航属性改造为外键ID和导航属性相结合的方式。照办：

public int AdminClassID { get; set; }        //老师作为班主任管理的班级        public SchoolClass AdminClass { get; set; }

再次运行，可以创建数据库了，但是报错：

SqlException: The INSERT statement conflicted with the FOREIGN KEY constraint "FK_Teacher_SchoolClass_AdminClassID". The conflict occurred in database "StudentWebContext", table "dbo.SchoolClass", column 'ID'.

大意是AdminClassID属性不允许为空。看数据库设计器Teacher的代码，AdminClassID是非空的：

CREATE TABLE [dbo].[Teacher] (    [ID]           INT            IDENTITY (1, 1) NOT NULL,    [Name]         NVARCHAR (MAX) NULL,    [AdminClassID] INT            NOT NULL,    CONSTRAINT [PK_Teacher] PRIMARY KEY CLUSTERED ([ID] ASC),    CONSTRAINT [FK_Teacher_SchoolClass_AdminClassID] FOREIGN KEY ([AdminClassID]) REFERENCES [dbo].[SchoolClass] ([ID]) ON DELETE CASCADE

实际上，一位老师，是可以不担当任何一个班级的班主任的，因此AdminClassID属性应该是可空的。再改一下

public int? AdminClassID { get; set; }

删除数据表，再次运行，没有任何问题了，数据库Teacher代码是正确的，

CREATE TABLE [dbo].[Teacher] (    [ID]           INT            IDENTITY (1, 1) NOT NULL,    [Name]         NVARCHAR (MAX) NULL,    [AdminClassID] INT            NULL,    CONSTRAINT [PK_Teacher] PRIMARY KEY CLUSTERED ([ID] ASC),    CONSTRAINT [FK_Teacher_SchoolClass_AdminClassID] FOREIGN KEY ([AdminClassID]) REFERENCES [dbo].[SchoolClass] ([ID])

Details页面数据显示也是正确的。