updating a database [* Resolved *]

Hello,

I am developing a library system. I have 2 tables, book and student. The relationship is 1 student can have many books. I am having updating the book table when the student returns the book, the studentID (foreign key) will be cleared from the book table. I think this is the problem l am having. My code is listed below with the error description.

Code:

---

try
                        {
                                cnnReturnBook.ConnectionString = @"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=C:\IBS Library System\LibrarySystem.mdb;Persist Security Info=False";
                       
                                //Find the book that have to be returned
                                OleDbCommand cmdReturnBook = cnnReturnBook.CreateCommand();
                                cmdReturnBook.CommandText = @"SELECT * FROM BOOK
                                                                                          WHERE RefNumber = '" + txtBookRef.Text + "' ";

                                OleDbDataAdapter daReturnBook = new OleDbDataAdapter(cmdReturnBook);
                                OleDbCommandBuilder cbReturnBook = new OleDbCommandBuilder(daReturnBook);

                                if ( daReturnBook.Fill(dtReturnBook) == 1 )//Book has been found, there should be only one 1 record in the datatable
                                {
                                        //Clear the text boxes
                                        txtDateOut.Text = "";
                                        txtDateDue.Text = "";
                                        txtStatus.Text = "Available"; //Show that the book is now available
                                        txtIDNumber.Text = ""; //ID number of the student who has this book.
                                        txtName.Text = "";
                                        txtSurname.Text = "";

                                        //Insert into the datatable row the new values
                                        dtReturnBook.Rows[0]["Status"] = txtStatus.Text;
                                        dtReturnBook.Rows[0]["DateOut"] = txtDateOut.Text;
                                        dtReturnBook.Rows[0]["Remarks"] = txtRemarks.Text;
                                        dtReturnBook.Rows[0]["StudentID"] = txtIDNumber.Text;
                                        dtReturnBook.Rows[0]["DateDue"] = txtDateDue.Text;

                                        daReturnBook.Update(dtReturnBook);//Error here
                                }
                        }
                        catch ( OleDbException ex )
                        {
                                MessageBox.Show(ex.ToString(),"Exeception has occurred",MessageBoxButtons.OK,MessageBoxIcon.Warning);
                        }
                        catch ( Exception ex )
                        {
                                MessageBox.Show(ex.ToString(),"Exeception has occurred",MessageBoxButtons.OK, MessageBoxIcon.Warning);
                        }
                        finally
                        {
                                cnnReturnBook.Dispose();
                        }
                }

---

Error description
System.Data.OleDb.OleDbException: You cannot add or change a record because a related record is required in table 'Student'.
At System.Data.Common.DbDataAdapter.Update(DataRow[] dataRows, DataTableMapping tableMapping)
At System.Data.Common.DbDataAdapter.Update(DataTable dataTable)

Thanks in advance,

Steve