Many to Many relation with Entity Framework in ASP.NET GridView in a simplified way

<%@ Page Title="Home Page" Language="C#" MasterPageFile="~/Site.master" AutoEventWireup="true"
    CodeBehind="Default.aspx.cs" Inherits="ManyToMany._Default" MetaKeywords="Title"
    MetaDescription="Entity Framework Example" %>

<asp:Content ID="HeaderContent" runat="server" ContentPlaceHolderID="HeadContent">
    <style type="text/css">
        .style1
        {
            width: 100%;
        }
    </style>
</asp:Content>
<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">
    <asp:Label runat="server" ID="lblMessage" ForeColor="Red"></asp:Label><br />
    <asp:Panel runat="server" ID="Pnl" BorderStyle="None" GroupingText="Add Product">
        <table class="style1">
            <tr>
                <td align="right">
                    Product Name
                </td>
                <td>
                    <asp:TextBox runat="server" ID="txtProd" EnableViewState="false" />
                    <asp:RequiredFieldValidator runat="server" ID="ReqProductName" ControlToValidate="txtProd"
                        ErrorMessage="Product Name required." ValidationGroup="AddProuct" ForeColor="Red"></asp:RequiredFieldValidator>
                </td>
            </tr>
            <tr>
                <td>
                </td>
                <td align="left">
                    <asp:Button ID="btnProd" Text="Add Product" runat="server" OnClick="btnProd_Click"
                        ValidationGroup="AddProuct" />
                </td>
            </tr>
        </table>
    </asp:Panel>
    <asp:Panel runat="server" ID="Panel1" BorderStyle="None" GroupingText="Add Customer">
        <table class="style1">
            <tr>
                <td align="right">
                    Customer Name
                </td>
                <td>
                    <asp:TextBox ID="txtCus" runat="server" EnableViewState="false" />
                    <asp:RequiredFieldValidator runat="server" ID="ReqCus" ControlToValidate="txtCus"
                        ErrorMessage="Customer Name required." ValidationGroup="AddCustomer" ForeColor="Red"></asp:RequiredFieldValidator>
                </td>
            </tr>
            <tr>
                <td align="right">
                    Select Product
                </td>
                <td>
                    <asp:DropDownList runat="server" ID="drpProd" AppendDataBoundItems="true">
                        <asp:ListItem Value="">--Select--</asp:ListItem>
                    </asp:DropDownList>
                    <asp:RequiredFieldValidator runat="server" ID="ReqProd" ControlToValidate="drpProd"
                        ErrorMessage="Please select Product Name." ValidationGroup="AddCustomer" ForeColor="Red"></asp:RequiredFieldValidator>
                </td>
            </tr>
            <tr>
                <td>
                </td>
                <td align="left">
                    <asp:Button ID="btnCust" Text="Add Customer" runat="server" OnClick="btnCust_Click"
                        ValidationGroup="AddCustomer" />
                </td>
            </tr>
        </table>
    </asp:Panel>
    <asp:Panel runat="server" ID="Panel2" BorderStyle="None" GroupingText="Customer & Product Details">
        <table class="style1">
            <tr>
                <td align="left">
                    <asp:GridView runat="server" ID="grdDetails" OnRowDeleting="grdDetails_RowDeleting"
                        OnRowEditing="grdDetails_RowEditing" 
                        OnRowUpdating="grdDetails_RowUpdating" OnRowCancelingEdit="grdDetails_RowCancelingEdit"
                        AutoGenerateColumns="false" onrowdatabound="grdDetails_RowDataBound"   >
                        <Columns>
                                                       
                              <asp:TemplateField>
                                <HeaderTemplate>
                                    Customer Id
                                </HeaderTemplate>
                                <ItemTemplate>
                                <asp:Label runat="server" ID="lblCusId" Text='<%# Eval("CustomerId") %>'    ></asp:Label>
                                </ItemTemplate>
                            </asp:TemplateField>

                            <asp:BoundField DataField="CustName" HeaderText="Customer Name" NullDisplayText="This field is required." />
                          


                              <asp:TemplateField>
                                <HeaderTemplate>
                                    Product Id
                                </HeaderTemplate>
                                <ItemTemplate>
                                <asp:Label runat="server" ID="lblPId" Text='<%# Eval("ProductId") %>' ></asp:Label>
                                </ItemTemplate>
                            </asp:TemplateField>


                            <asp:TemplateField>
                                <HeaderTemplate>
                                    Product Name
                                </HeaderTemplate>
                                <ItemTemplate>
                                      <asp:DropDownList runat="server" ID="drpProd1" AppendDataBoundItems="true" Enabled="false">
                                        <asp:ListItem Value="">--Select--</asp:ListItem>
                                     </asp:DropDownList>
                                </ItemTemplate>
                            </asp:TemplateField>
                            <asp:CommandField ShowEditButton="true" ShowCancelButton="true" ShowDeleteButton="true"
                                CancelText="Cancel" DeleteText="Delete" UpdateText="Update" HeaderText="Action" />
                        </Columns>
                        <EmptyDataTemplate>
                            No Customer & Product details avaliable.</EmptyDataTemplate>
                    </asp:GridView>
                </td>
            </tr>
        </table>
    </asp:Panel>
</asp:Content>