public IHttpActionResult PutPromotion(int id, Promotion promotion)
        {
            if (!ModelState.IsValid)
            {
                return BadRequest(ModelState);
            }

            if (id != promotion.Id)
            {
                return BadRequest();
            }

            db.Entry(promotion).State = EntityState.Modified;

            try
            {
                db.SaveChanges();
            }
            catch (DbUpdateConcurrencyException)
            {
                if (!PromotionExists(id))
                {
                    return NotFound();
                }
                else
                {
                    throw;
                }
            }

            return StatusCode(HttpStatusCode.NoContent);
        }
        public IHttpActionResult PostPromotion(Promotion promotion)
        {
            if (!ModelState.IsValid)
            {
                return BadRequest(ModelState);
            }

            db.Promotions.Add(promotion);
            db.SaveChanges();

            return CreatedAtRoute("DefaultApi", new { id = promotion.Id }, promotion);
        }