I am getting an error adding data to a table that is many to many relationship

Question

There is a many to many relationship table. I get an error adding data to it.

Error:The relationship between the two objects cannot be defined because they are attached to different ObjectContext objects

    public class Student
    {
        public Student()
        {
            this.isPassive = false;
            Lessons = new HashSet<Lesson>();
        }
        public int ID { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public string FullName { get { return FirstName + " " + LastName; } }
        public string Mail { get; set; }
        public string Password { get; set; }
        public Nullable<int> LessonCredit { get; set; }
        public bool isPassive { get; set; }
        public int AccountTypeID { get; set; }

        //Navigation Property

        public virtual AccountType AccountType { get; set; }
        public virtual ICollection<Lesson> Lessons { get; set; }
    }

   public class Lesson
    {
        public Lesson()
        {
            Students = new HashSet<Student>();
            this.isPassive = false;
        }
        public int ID { get; set; }
        public string Name { get; set; }
        public int Credit { get; set; }
        public bool isPassive { get; set; }
        public Nullable<int> TeacherID { get; set; }

        //Navigation Property
        public virtual Teacher Teacher { get; set; }
        public virtual ICollection<Student> Students { get; set; }

    }

     [HttpPost]
        public ActionResult LessonSelection(int[] LessonChecked)
        {
            int loginStudentId = Convert.ToInt32(Session["PersonId"]);
            Student loginStudent = _studentBLL.Get(loginStudentId);

            for (int i = 0; i < LessonChecked.Length; i++)
            {
                Lesson lesson = _lessonBLL.Get(LessonChecked[i]);
                loginStudent.Lessons.Add(lesson);
                _studentBLL.Update(loginStudent);
            }
            return View();
        }