public HttpResponseMessage ApproveRental([FromBody] Rental rental)
        {
            string currentUserEmailAddress;

            if (Request.Headers.GetValues("EmailId").Count() == 0)
            {
                return(new HttpResponseMessage()
                {
                    StatusCode = HttpStatusCode.Unauthorized
                });
            }

            currentUserEmailAddress = Request.Headers.GetValues("EmailId").First();

            // validate if user is an admin
            if (!accountService.CheckIfGivenEmailIsOfAdmin(currentUserEmailAddress))
            {
                return(new HttpResponseMessage()
                {
                    StatusCode = HttpStatusCode.Unauthorized
                });
            }

            rentalService.ApproveRental(rental.UserName, rental.BookName);

            return(new HttpResponseMessage()
            {
                StatusCode = HttpStatusCode.OK
            });
        }