Entity framework 3.5 foreign key missing table mapping

 var Budgetlinevalue=Convert.ToInt32(dropdownbudgetline.SelectedValue);

 MstrBudgetLine budgetlineID = db.MstrBudgetLines.Where(x => x.BudgetLinesPkID == Budgetlinevalue).FirstOrDefault();

            Initiative tbl = new Initiative();

  tbl.MstrBudgetLine = budgetlineID;
 db.AddToInitiatives(tbl);
 db.SaveChanges();