public static bool AddTeacher(Teacher teacher)
        {
            var query = Query.EQ("TeaNo", teacher.TeaNo);

            if (collection.Find(query).Count() > 0) return false;
            else
            {
                collection.Insert<Teacher>(teacher);
                return true;
            }
        }
 public static void Update(Teacher teacher)
 {
     collection.Update(new QueryDocument { { "_id", teacher.Id } },
         new UpdateDocument { { "$set",
                                  new BsonDocument {
                                  {"TeaNo", teacher.TeaNo },
                                  {"Name", teacher.Name},
                                  {"Institution", teacher.Institution}
                                  }}});
 }