public IActionResult JoinGroup(int id) { var group = _groupHandler.GetById(id); var user = _userManager.GetUserAsync(User).Result; if (user.UsersInGroup == null) { user.UsersInGroup = new List <UsersInGroup>(); } if (_usersInGroupHandler.GetUserInGroup(id, user.Id) == null) { _usersInGroupHandler.AddUserInGroup(group, user.Id); return(RedirectToAction("Groups", "Chat", new { id })); } else { ModelState.AddModelError("JoinGroup", "You already joined"); return(RedirectToAction("Groups", "Chat", new { id })); } }