public Task<Postcode> GetOne(string id) =>

         _context.Postcode.Include(p => p.DataZoneNavigation).AsNoTracking().FirstOrDefaultAsync(m => m.Postcode1 == id);

public  class Postcode
    {
        public string Postcode1 { get; set; }
        public string DataZone { get; set; }

        public virtual Data DataZoneNavigation { get; set; }
    }

Task<Postcode> GetOne(string id);

// GET: Postcodes/Details/5
        public async Task<IActionResult> Details(string id)
        {
            if (id == null)
            {
                return NotFound();
            }

            var postcode = await _postcodesRepository.GetOne(id);

            if (postcode == null)
            {
                return NotFound();
            }

            return View(postcode);
        }