19

我正在尝试在 EF 中进行测试，创建多对多关系，因为我总是映射一对一或一对多，我在互联网上有一个示例可供尝试，该示例适用于插入寄存器，但我无法读取寄存器

这是我的课程，我不知道是什么HashSet，我在网站上得到了这个代码

public class Person
{
    public int PersonId { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }

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

    public Person()
    {
        CoursesAttending = new HashSet<Course>();
    }
}

public class Course
{
    public int CourseId { get; set; }
    public string Title { get; set; }

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

    public Course()
    {
        Students = new HashSet<Person>();
    }
}

这是我的上下文

public class SchoolContext : DbContext
{
    public DbSet<Course> Courses { get; set; }
    public DbSet<Person> People { get; set; }

    public SchoolContext()
        : base("MyDb")
    {
    }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Course>().
            HasMany(c => c.Students).
            WithMany(p => p.CoursesAttending).
            Map(
                m =>
                {
                    m.MapLeftKey("CourseId");
                    m.MapRightKey("PersonId");
                    m.ToTable("PersonCourses");
                });

    }
}

当我插入寄存器是正确的

static void Main(string[] args)
{
    using (SchoolContext db = new SchoolContext())
    {
        Course math = new Course();
        Course history = new Course();
        Course biology = new Course();
        math.Title = "Math";
        history.Title = "History";
        biology.Title = "Biology";

        db.Courses.Add(math);
        db.Courses.Add(history);
        db.Courses.Add(biology);

        Person john = new Person();
        john.FirstName = "John";
        john.LastName = "Paul";
        john.CoursesAttending.Add(history);
        john.CoursesAttending.Add(biology);

        db.People.Add(john);
        db.SaveChanges();
    }
}

但是当我尝试为显示内容选择注册时，它不起作用，它什么也不打印

static void Main(string[] args)
{
    using (SchoolContext db = new SchoolContext())
    {
        Pearson p = db.Peasons.First();
        Console.WriteLine(p.CoursesAttending.First().Title);
    }
}

我在数据库中检查过，寄存器存在，是什么问题？

请先教我如何在与代码的多对多关系中进行选择。

4

2 回答 2

34

First off, you may want to enable lazy loading by making the collections virtual:

public virtual ICollection<Course> CoursesAttending { get; set; }
public virtual ICollection<Person> Students { get; set; }

This allows EF to create derived classes (proxies) from Course and Person that override the collections with logic to load their data from the data store.

When you do that you will see that 

Console.WriteLine(p.CoursesAttending.First().Title);

will execute a separate query to populate CoursesAttending.

Alternatively, or additionally, you may decide to prevent round trips to the database by eager loading like so:

Person p = db.Persons.Include(p => p.CoursesAttending).First();

Which loads the Person and the CoursesAttending in one shot.

于 2013-10-28T23:50:26.463 回答
-1

Remove your class constructor and that OnModelCreating method. EF handles Many to Many relationship. You don't need to do anything extra.

于 2013-10-28T23:49:10.797 回答