代码第一实体框架多对多关系 [英] Code First Entity Framework Many-to-Many relationships

问题描述

有人可以指出我错了什么！

我已经创建了两个简单的类，具有多对多的关系。工作正常，所有的表都正确填充。除了当我尝试和撤销任何学生课程没有任何回报...

  public partial class Student 
 {
 
 public Student（）
 {
课程= new HashSet< Course>（）; 
} 
 
 public int StudentID {get;组; 
 
 [StringLength（50）] 
 [必需] 
 public string FirstName {get; set;} 
 
 [StringLength（50）] 
 [必需] 
 public string LastName {get; set;} 
 
 public DateTime？ Enroled {get; set;} 
 
 public ICollection< Course>课程{get;组; } 
 
 
} 
 
 public class课程
 {
 
 public Course（）
 {
学生=新的HashSet< Student>（）; 
} 
 public int CourseID {get;组; 
 
 [StringLength（30）] 
 [必需] 
 public string CourseTitle {get;组; } 
 
 [StringLength（255）] 
 public string CourseDesc {get;组; } 
 
 public ICollection< Student>学生{get;组; 
} 
 
 public ContextDB（）
：base（@Data Source = .\SQLEXPRESS; Initial Catalog = CollegeDB; Integrated Security = True）
 {
 
} 
 
 protected override void OnModelCreating（DbModelBuilder modelBuilder）
 {
 modelBuilder.Entity< Course>（）。HasMany（c => c.Students）.WithMany（p => p.Courses）
 .Map（
m => {
 m.MapLeftKey（Course_CourseID）; 
 m.MapRightKey （Student_StudentID）; 
 m.ToTable（CourseStudents）; 
}）; 
} 
 
 public DbSet< Student>学生{get;组; } 
 public DbSet< Course>课程{get;组; } 
 
 
} 
 
 
 
 var students = db.Students.ToList（）; 
 
 foreach（学生在学生中）
 {
 Console.WriteLine（{0} {1}，s.FirstName，s.LastName）; 
 foreach（s.Courses.ToList（）中的课程c）
 {
 Console.WriteLine（{0}，c.CourseTitle）; 
} 
} 
  

解决方案

您正在使用延迟加载，您需要将属性定义为虚拟，

  public virtual ICollection< Course>课程{get;组; 
  

如果您希望可以使用热切加载，

  var students = db.Students.Include（s => s.Courses）.ToList（）; 
  

http://codetuner.blogspot.com/2011/07/entity-framework-differed-loading-lazy.html

Can someone point out where I've got wrong!!

I have created 2 simple classes, with many-to-many relationship. Works fine, all the tables are populated correctly. Except when I try and retrive any Students courses nothing is returned...

public partial class Student 
{

    public Student()
    {
        Courses = new HashSet<Course>();
    }

    public int StudentID { get; set; }

    [StringLength(50)] 
    [Required]
    public string FirstName { get; set;}

    [StringLength(50)]
    [Required]
    public string LastName {get; set;}

    public DateTime? Enroled {get;set;}

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


}

public class Course
{

    public Course()
    {
        Students = new HashSet<Student>();
    }
    public int CourseID { get; set; }

    [StringLength(30)]
    [Required]
    public string CourseTitle { get; set; }

    [StringLength(255)]
    public string CourseDesc { get; set; }

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

    public ContextDB()
        : base (@"Data Source=.\SQLEXPRESS;Initial Catalog=CollegeDB;Integrated Security=True")
    {

    }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Course>().HasMany(c=>c.Students).WithMany(p => p.Courses)
            .Map(
            m=> {
                m.MapLeftKey("Course_CourseID");
                m.MapRightKey("Student_StudentID");
                m.ToTable("CourseStudents");
            });
    }

    public DbSet<Student> Students { get; set; }
    public DbSet<Course> Courses { get; set; }


}



          var students = db.Students.ToList();

            foreach (Student s in students)
            {
                Console.WriteLine("{0} {1}", s.FirstName, s.LastName);
                foreach (Course c in s.Courses.ToList())
                {
                    Console.WriteLine("{0}", c.CourseTitle);
                }
            }

解决方案

If you are using lazy loading you need to defiine properties as virtual ,

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

and if you want you can use eager loading,

var students = db.Students.Include(s=>s.Courses).ToList();

http://codetuner.blogspot.com/2011/07/entity-framework-differed-loading-lazy.html

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