根据文档，EF Core 5 多对多关系无法正常工作

Question

According to the documentation , EF Core 5.0 supports many-to-many relationships without explicitly mapping the join table.

I'm having issues with the below in EF Core 5.0.9:

public List<Booking> GetAllBookingsForDay(int day, DateTime bookingDate)
{
     return db.Bookings
    .Where(w => w.IsActive == true & w.IsDeleted == false & w.DayOfWeek == day & w.Time >= bookingDate)
    .Include(c => c.Students)
    .ToList();
}

I have included Students as part of the query.

Booking.cs

public class Booking
{
    public int Id { get; set; }
    public int? DayOfWeek { get; set; }
    public DateTime? BookingDate { get; set; }
    public bool? IsAbsent { get; set; }
    public DateTime? Time { get; set; }
    public bool? HasCheckedIn { get; set; }
    public ICollection<Student> Students { get; set; }  // notice this
    public int? StudentId { get; set; }
    public bool? IsDeleted { get; set; }
    public bool? IsActive { get; set; }
    public string? CreatedBy { get; set; }
    public string? LastModifiedBy { get; set; }
}

Student.cs

public class Student
{   
    public int Id { get; set; }
    public int? Type { get; set; }
    public int? TeamId { get; set; }
    public string? FirstName { get; set; }
    public string? Surname { get; set; }
    public DateTime? DOB { get; set; }
    public decimal? Weight { get; set; }
    public decimal? Height { get; set; }
    public int? Gender { get; set; }
    public string? Photo { get; set; }
    public int? Age { get; set; }
    public ICollection<Booking> Bookings { get; set; } // notice here my link back 
     to bookings
   ...
 }

Nuget packages:

SQL database:

When I inspect the booking table, its empty does EF not automatically add into BookingStudent if so how do I tell ef core to add in the student? I have saved their StudentId into booking so why has it not created an entry.

It Created the booking table entry

In My head I should see the values 10 and 72 in booking student should I not.

But not the booking Student Entry?. According to the docs it creates it automatically yeah but what about saving entries does it not still do that automatically.

At this stage I have not placed anything in my DBContext on create method as the docs say u dont need that any more???

This is how I save my student. As you see am filling in the foreign key of StudentId.

Booking newBooking = new Booking();
newBooking.DayOfWeek = DayNumber;
newBooking.BookingDate = BookingDate;
newBooking.Time=Helpers.Dates.GetDateZeroTime(selectedBookingDate.Date).Add(timePicker.Time);
newBooking.StudentId = StudentId;
newBooking.HasCheckedIn = false;
newBooking.IsActive = true;
newBooking.IsDeleted = false;
newBooking.IsAbsent = false;
await api.AddToBooking(newBooking);
await DisplayAlert(Constants.AppName, "Booking Created For Student", "OK");

My Add to booking which the api is calling in the web api is.

public Booking AddBooking(Booking  booking)
{
    db.Bookings.Add(booking);
    db.SaveChanges();
    return booking;
}

Edit 2

Is this what u mean

//Do i need to put the second param in for Student here ?
public Booking AddBooking(Booking booking, Student student)
{
        booking.Students.Add(student);
        db.Bookings.Add(booking);

        db.SaveChanges();
        return booking;
}

Answer 1

If it's a many-to-many relationship, why do you have a StudentId property on Booking ?

Remove the property first:

public class Booking
{
    public int Id { get; set; }
    public int? DayOfWeek { get; set; }
    public DateTime? BookingDate { get; set; }
    public bool? IsAbsent { get; set; }
    public DateTime? Time { get; set; }
    public bool? HasCheckedIn { get; set; }
    public ICollection<Student> Students { get; set; }
    public bool? IsDeleted { get; set; }
    public bool? IsActive { get; set; }
    public string? CreatedBy { get; set; }
    public string? LastModifiedBy { get; set; }
}

And then add the student you want (which will have an ID) to the ICollection of students you have for a booking.

booking.Students.Add(student);

That will trigger the EF many-many relationship.

Something like this:

var student = new Student { ... };

var newBooking = new Booking {
    Student = new List<Student> { student },
    ...
};

await api.AddToBooking(newBooking);