Mapping many to many relationship in entity framework code first

Question

I'm try make a test in EF, creating a many to many relationship, because I always mapping One to One or One to Many, I has get a example in the internet for try, the example is working for insert registers, but I can't read registers

Here is my classes, I don't know what is HashSet , I get this code in the site

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>();
    }
}

Here is my Context

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");
                });

    }
}

When I insert registers is right

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();
    }
}

But when I try select register for show content, is not work, it just print nothing

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

I had check in the database, registers exist, what is the problem?

Please teach me how to make select in many to many relationship with code first.

Answer 1

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.

Answer 2

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