EF6 多对多表中没有数据

Question

我正在使用Getting Started with EF 6 using MVC 5 教程（Contoso 大学示例应用程序）学习 ASP.NET MVC 和 EF。这是我在这里的第一个问题，如果问题的形式不完美，请原谅我（告诉我有什么问题）。

我一直坚持多对多关系。表已创建，但里面没有数据。

我已经为两个类添加了虚拟导航属性，并在 SchoolContext 中设置了映射。

Course.cs

 public class Course
    {
        [DatabaseGenerated(DatabaseGeneratedOption.None)]
        [Display(Name = "Numer")]
        public int ID { get; set; }

        [StringLength(50, MinimumLength = 3)]
        [Display(Name = "Przedmiot")]
        public string Title { get; set; }

        [Range(0, 5)]
        [Display(Name = "ECTS")]
        public int Credits { get; set; }

        public int DepartmentID { get; set; }

        public virtual Department Department { get; set; }
        public virtual ICollection<Enrollment> Enrollments { get; set; }
        public virtual ICollection<Instructor> Instructors { get; set; }
    }

Instructor.cs

public class Instructor
    {
        public int ID { get; set; }

        [Required]
        [Display(Name = "Nazwisko")]
        [StringLength(50)]
        public string LastName { get; set; }

        [Required]
        [Column("FirstMidName")]
        [Display(Name = "Imię")]
        [StringLength(50)]
        public string FirstName { get; set; }

        [DataType(DataType.Date)]
        [DisplayFormat(DataFormatString = "{0:yyyy-MM-dd}", ApplyFormatInEditMode = true)]
        [Display(Name = "Data zatrudnienia")]
        public DateTime HireDate { get; set; }

        public string FullName 
        { 
            get
            {
                return LastName + " " + FirstName;
            }
        }

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

SchoolContext.cs

public class SchoolContext : DbContext
    {
        public SchoolContext() : base("SchoolContext")
        {
        }
        public DbSet<Course> Courses { get; set; }
        public DbSet<Department> Departments { get; set; }
        public DbSet<Enrollment> Enrollments { get; set; }
        public DbSet<Instructor> Instructors { get; set; }
        public DbSet<Student> Students { get; set; }
        public DbSet<OfficeAssignment> OfficeAssignments { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();
            modelBuilder.Entity<Course>()
              .HasMany(c => c.Instructors).WithMany(i => i.Courses)
              .Map(t => t.MapLeftKey("CourseID")
                  .MapRightKey("InstructorID")
                  .ToTable("CourseInstructor"));
        }
    } 

数据在 Migrations/Configuration.cs 中的 Seed() 方法中初始化：

protected override void Seed(MyUniversity.DAL.SchoolContext context)
{ (...) 
var instructors = new List<Instructor>
            {
                new Instructor { FirstName = "Antoni", LastName = "Czerwiński", HireDate = DateTime.Parse("2016-07-01")},
                new Instructor { FirstName = "Czesław", LastName = "Piotrowski", HireDate = DateTime.Parse("2016-07-01")},
                new Instructor { FirstName = "Paweł", LastName = "Andrzejewski", HireDate = DateTime.Parse("2016-07-01")},
                new Instructor { FirstName = "Karol", LastName = "Sokołowski", HireDate = DateTime.Parse("2016-07-01")},
                new Instructor { FirstName = "Alojzy", LastName = "Woźniak", HireDate = DateTime.Parse("2016-07-01")}
            };
            instructors.ForEach(s => context.Instructors.AddOrUpdate(p => p.LastName, s));
            context.SaveChanges();

var courses = new List<Course>
            {
                new Course { ID = 1050, Title = "Chemia", Credits =3,
                    DepartmentID = departments.Single(s => s.Name == "Inżynierii").ID, Instructors = new List<Instructor>() },
                new Course { ID = 4022, Title = "Mikroekonomia", Credits =3,
                    DepartmentID = departments.Single(s => s.Name == "Ekonomiczny").ID, Instructors = new List<Instructor>() },
                new Course { ID = 4041, Title = "Makroekonomia", Credits =3,
                    DepartmentID = departments.Single(s => s.Name == "Ekonomiczny").ID, Instructors = new List<Instructor>() },
                new Course { ID = 1045, Title = "Rachunek Różniczkowy", Credits =4,
                    DepartmentID = departments.Single(s => s.Name == "Matematyki").ID, Instructors = new List<Instructor>() },
                new Course { ID = 3141, Title = "Trygonometria", Credits =4,
                    DepartmentID = departments.Single(s => s.Name == "Matematyki").ID, Instructors = new List<Instructor>() },
                new Course { ID = 2021, Title = "Kompozycja", Credits =3,
                    DepartmentID = departments.Single(s => s.Name == "Anglistyki").ID, Instructors = new List<Instructor>() },
                new Course { ID = 2042, Title = "Literatura", Credits =4,
                    DepartmentID = departments.Single(s => s.Name == "Anglistyki").ID, Instructors = new List<Instructor>() }
            };
            courses.ForEach(s => context.Courses.AddOrUpdate(p => p.ID, s));
            context.SaveChanges();  
(...)
}

DbTablesRelationsImage

DbCourseInstructorTableImage

如果需要，这里是我的 github 上的项目链接：MyGithub

编辑(14:15)

我在 Seed 中有将教师分配给课程的方法。 但问题是为什么表格中仍然没有数据？

protected override void Seed(MyUniversity.DAL.SchoolContext context)
{ (...) 
            AddOrUpdateInstructor(context, "Chemia", "Sokołowski");
            AddOrUpdateInstructor(context, "Chemia", "Andrzejewski");
            AddOrUpdateInstructor(context, "Mikroekonomia", "Woźniak");
            AddOrUpdateInstructor(context, "Makroekonomia", "Woźniak");

            AddOrUpdateInstructor(context, "Rachunek Różniczkowy", "Piotrowski");
            AddOrUpdateInstructor(context, "Trygonometria", "Andrzejewski");
            AddOrUpdateInstructor(context, "Kompozycja", "Czerwiński");
            AddOrUpdateInstructor(context, "Literatura", "Czerwiński");

            context.SaveChanges();
}

void AddOrUpdateInstructor(SchoolContext context, string courseTitle, string instructorName)
        {
            var crs = context.Courses.SingleOrDefault(c => c.Title == 
                courseTitle);
            var inst = context.Instructors.SingleOrDefault(i => i.LastName == 
                instructorName);
            if (inst == null)
            {
                crs.Instructors.Add(context.Instructors.Single(i => i.LastName == 
                instructorName));
            }
        }

Answer 1

在播种期间，您绝不会为课程分配讲师；每门课程都有一个空的讲师列表。因此，除非您的问题中缺少这一点，否则这就是多对多表为空的原因。

Answer 2

默认情况下，导航属性不加载查询。

例如：

using ( var context = new SchoolContext () ) {
    var professor = context.Instructors.FirstOrDefault();
    var courses = professor.Courses // this will be null even if there are linked courses.  
}

要解决此问题，您可以使用 Include()。

例如：

using ( var context = new SchoolContext () ) {
    var professor = context.Instructors
        // this instructs EF to do the Join operation and include the Courses navigation property. 
        .Include ( prof => prof.Courses )   
        .FirstOrDefault();
    var courses = professor.Courses // this will now be populated with courses.  
}