public async System.Threading.Tasks.Task<int> AddSensorData(sensor_data sensor_data)
        {
            sensor_data.timestamp = DateTime.Now;

            db.sensor_data.Add(sensor_data);
            return await db.SaveChangesAsync();

        }
        public async Task<IHttpActionResult> Postsensor_data(sensor_data sensor_data)
        {
            if (!ModelState.IsValid)
            {
                return BadRequest(ModelState);
            }

            await db.AddSensorData(sensor_data);
            return CreatedAtRoute("DefaultApi", new { id = sensor_data.Id }, sensor_data);
        }