public HttpResponseMessage PostStudents(StudentDetails model)
        {
            List<Mark> newMarks = new List<Mark>();
            foreach (var mark in model.Marks)
            {
                var markEntity = new Mark()
                {
                    Subject = mark.Subject,
                    Value = mark.Value
                };

                newMarks.Add(markEntity);

            }
           
            var entityToAdd = new Student()
            {
                FirstName = model.FirstName,
                LastName = model.LastName,
                Age = model.Age,
                Grade = model.Grade,
                SchoolId = model.SchoolId,
                Marks = newMarks
            };
            var createdEntity = this.studentsRepository.Add(entityToAdd);

            var createdModel = new StudentDetails()
            {
                Id = createdEntity.Id,
                FirstName = createdEntity.FirstName,
                LastName = createdEntity.LastName,
                Grade = createdEntity.Grade,
                Age = createdEntity.Age,
                SchoolId = createdEntity.SchoolId
            };

            

            var response = Request.CreateResponse<StudentDetails>(HttpStatusCode.Created, createdModel);
            var resourceLink = Url.Link("DefaultApi", new { id = createdModel.Id });

            response.Headers.Location = new Uri(resourceLink);
            return response;
        }
        public StudentDetails GetSingle(int id)
        {
            var entity = this.studentsRepository.Get(id);
            var model = new StudentDetails()
            {
                Id = entity.Id,
                FirstName = entity.FirstName,
                LastName = entity.LastName,
                Age = entity.Age,
                Grade = entity.Grade,
                SchoolId = entity.SchoolId,
                Marks = (from mark in entity.Marks
                         select new MarkModel()
                         {
                             Subject = mark.Subject,
                             Value = mark.Value
                         }
                        ).ToList()
            };

            return model;
        }