updating item from an external list

Question

I'm trying to update an item in my external list i'm using BCS and LINQ to query. That's my Update method :

public static void Update(Customer customer, int parameter)
        {
            CustomersDataContext dataContext = new CustomersDataContext(CONNECTION_STRING);
            Customer oldCustomer = (from c in dataContext.Customers.AsEnumerable()
                                 where c.CustomerID == customer.CustomerID
                                 select c).Single();

            //oldCustomer.CustomerID = customer.CustomerID;
            oldCustomer.FirstName = customer.FirstName;
            oldCustomer.LastName = customer.LastName;
            oldCustomer.TwitterScreenName = customer.TwitterScreenName;
            oldCustomer.DateLastPurchase = customer.DateLastPurchase;
            oldCustomer.AmountLastPurchase = customer.AmountLastPurchase;

            dataContext.SubmitChanges();
        }

but nothing happens in my database PS: i'working on sharepoint 2010 and i created my entity using visual studio 2010

please help