Hi abmkp,
You need to add the column pkCityId to the DataKeyNames property of the grid as per below;
<asp:gridview id="GridView1" runat="server" autogeneratecolumns="False" allowpaging="true" OnPageIndexChanging="gdv1_OnPaging" PageSize="5" DataKeyNames = "Material,pkCityId" OnRowDataBound = "gv1_OnRowDataBound" OnSelectedIndexChanged = "gv1_SelectedIndexChanged" CSSClass= "~/Styles" EnableViewState="False" PagerSettings-Mode="Numeric" AutoGenerateSelectButton="true"</asp:gridview>
Kind Regards