thắc mắc Transaction trong EntityFramework

        [HttpPost]
        [Route("CreateQuotation")]
        public async Task<IHttpActionResult> CreateQuotation(Quotation quo)
        {
            System.Data.Entity.DbContextTransaction transaction = db.Database.BeginTransaction();
            try
            {
                var id = await quo.CreateQuotation();
                quo.CreateDetails(id);
                transaction.Commit();
                return Ok("Tạo thành công");
            }
            catch(Exception ex)
            {
                transaction.Rollback();
                return Ok(ex);
            }
        }

public class Quotation
    {
        private PivotdataEntities db = new PivotdataEntities();
        public QUOTATION quotation { set; get; }
        public List<QUOTATION_DETAILS> details { set; get; }
        public async Task<int> CreateQuotation()
        {
            QUOTATION newq = new QUOTATION();
            newq = this.quotation;
            newq.CREATE_DATE = DateTime.Now;
            newq.QUOTATION_CODE = CreateQuotationCode("HN");
            db.QUOTATIONs.Add(newq);
            await db.SaveChangesAsync();
                
            return newq.ID;
        }

        public void CreateDetails(int id)
        {
            foreach (var item in this.details)
            {
                QUOTATION_DETAILS newdetail = new QUOTATION_DETAILS();
                newdetail = item;
                newdetail.QUOTATION_ID = id;
                db.QUOTATION_DETAILS.Add(newdetail);
                db.SaveChanges();
            }
        }

        public string CreateQuotationCode(string macongty)
        {
            string thamso = "";
            if (macongty == "HN")
            {
                thamso = "BGHN";
            }
            else
                if (macongty == "HCM")
            {
                thamso = "BGHCM";
            }
            Regex digitsOnly = new Regex(@"[^\d]");
            string year = DateTime.Now.Year.ToString().Substring(2, 2);
            string month = DateTime.Now.Month.ToString();
            string day = DateTime.Now.Day.ToString();
            if (month.Length == 1)
            {
                month = "0" + month;
            }
            if (day.Length == 1)
            {
                day = "0" + day;
            }

            string prefixNumber = thamso + year.ToString() + month.ToString() + day.ToString();
            string SoChungTu = (from quo in db.QUOTATIONs where quo.QUOTATION_CODE.Contains(prefixNumber) select quo.QUOTATION_CODE).Max();


            if (SoChungTu == null)
            {
                return thamso + year + month + day + "0001";
            }
            SoChungTu = SoChungTu.Substring(10, SoChungTu.Length - 10);
            string number = (Convert.ToInt32(digitsOnly.Replace(SoChungTu, "")) + 1).ToString();
            string result = number.ToString();
            int count = 4 - number.ToString().Length;
            for (int i = 0; i < count; i++)
            {
                result = "0" + result;
            }
            return thamso + year + month + day + result;
        }
    }

System.Data.Entity.DbContextTransaction transaction = db.Database.BeginTransaction();
            var id = null;
            Boolean aaa = true;
            try
            {
                id = await quo.CreateQuotation();
                
                transaction.Commit();
                return Ok("Tạo thành công");
            }
            catch(Exception ex)
            {
                aaa = false;
                transaction.Rollback();
                return Ok(ex);
            }
            
            If (aaa && id != null) {
                quo.CreateDetails(id);
            }

private ApplicationDbContext _dbContext;

public UnitOfWork(ApplicationDbContext context)
{
   _dbContext = context;
}

public int CreateQuation(){
     using (var context1 = _dbContext.Database.BeginTransaction())
      {
             //handle transaction
      }
}