EF：删除带有桥表的对象

Question

I'm trying to keep track of attendances and students. For this I am using a bridge table. Students has an ICollection of PersonAttendances (bridgetable and bridgeclass), and Attendance also has an ICollection of personAttendances.

When a person attends a course on a certain date(property of attendance), in the attendance class the addStudent method gets called

public void AddPerson(Person person) {
    PersonAttendance personAttendance = new PersonAttendance {   
        Person = person, Attendance = this, 
        AttendanceId = this.AttendanceId, 
        PersonId = person.PersonId 
    };  
    PersonAttendances.Add(personAttendance);
}

So far, this seems to work, but I'm struggling to understand how to write a delete method, where the 'link' between a student and his attendance gets removed.

Someone in my group came up with this, but we are positive it wouldn't work, but this is the best we could come up with

public void DeletePersoon(Person person) {
    PersonAttendance personAttendance = new PersonAttendance {
        Person = person,
        Attendance = this,
        AttendanceId = this.AttendanceId,
        PersonId = person.PersonId
    };
    PersonAttendance.Remove(personAttendance);
}

Answer 1

First you need to read the desired object from database and then try remove it.
If you have an Entity for PersonAttendance (apparently your case):

var pa = db.PersonAttendance.Where(p => p.PersonId == 1 && p.AttendenceId == 5).FirstOrDefault();
db.PersonAttendance.Remove(pa);
db.SaveChanges();

Otherwise:

var att = db.Attendance.Where(a => a.Id == 5).FirstOrDefault();
person.Attendence.Remove(att)
db.SaveChanges();

Answer 2

If the primary key of PersonAttendance is a composite key using PersonId and AttendanceId , you can avoid loading the entity first by attaching an object with the PK properties set and then set the entity state to deleted:

var theCondemned = new PersonAttendance()
{
    PersonId = person.PersonId,
    AttendanceId = this.AttendanceId,
};

PersonAttendances.Attach( theCondemned )
    .State = EntityState.Deleted;

Otherwise @rad is correct - you must first load the entity from DB (to get the PK) to then remove it.