Sorting asp.net GridView with SPDataSource

http://stackoverflow.com/questions/16027310/sorting-asp-net-gridview-with-spdatasource

Then I created my grid, based on the datasource

    <asp:GridView Visible="true" Width="100%"
                ID="gvInventar"
                AutoGenerateColumns="false" runat="server" CellPadding="2" AllowPaging="false" AllowSorting="true" OnSorted="gvInventar_Sorted" OnSorting="gvInventar_Sorting" GridLines="None" DataSourceID="SPdsInventarGrid" OnRowCommand="gvInventar_RowCommand">
                <Columns>
                    <asp:CommandField ButtonType="Image" ShowEditButton="true" EditImageUrl="/_layouts/images/edit.gif" UpdateImageUrl="/_layouts/images/save.gif" CancelImageUrl="/_layouts/images/delete.gif" />

                    <asp:TemplateField HeaderText="ID">
                        <ItemTemplate>
                            <asp:Label Text='<%# Bind("ID") %>' runat="server" ID="lblIdProduct"></asp:Label>
                        </ItemTemplate>
                    </asp:TemplateField>
                    <asp:TemplateField HeaderText="Title" >
                        <ItemTemplate>
                            <asp:Label Text='<%# Bind("Title") %>' runat="server" ID="lblTitleProduct" ></asp:Label>
                        </ItemTemplate>
                    </asp:TemplateField>
                    <asp:TemplateField HeaderText="Product" SortExpression="Product">
                        <ItemTemplate>
                            <asp:Label Text='<%# Bind("EBS_x002e_CN_x002e_Inventar_x0028") %>' runat="server" ID="lblProduct" "></asp:Label>
                        </ItemTemplate>

                </Columns>
    </asp:GridView>

Now, I have this function where I wanted to get the current Query as a string and add <OrderBy>(my field) before <Query> but i get an error saying that the DataSource has been already declared (which is true because i want to be declared in asp)

protected void gvInventar_Sorting(object sender, GridViewSortEventArgs e)
    {
        SPQuery q = new SPQuery();
        q.Query = SPdsInventarGrid.SelectCommand;
        switch (e.SortExpression)
        {
            case "Product":
                if (e.SortDirection == SortDirection.Ascending)
                {
                    gvInventar.DataSource = q.Query;
                    gvInventar.DataBind();
                }
                else
                {
                }
               break;
        }
    }

<Columns>
    <asp:TemplateField HeaderText="ID" SortExpression="ID">
        <ItemTemplate>
            <asp:Label Text='<%# Bind("ID") %>' runat="server" ID="lblIdProduct" />
        </ItemTemplate>
    </asp:TemplateField>
    <asp:TemplateField HeaderText="Title" SortExpression="Title">
        <ItemTemplate>
            <asp:Label Text='<%# Bind("Title") %>' runat="server" ID="lblTitleProduct" />
        </ItemTemplate>
    </asp:TemplateField>
    <asp:TemplateField HeaderText="Product" SortExpression="EBS_x002e_CN_x002e_Inventar_x0028">
        <ItemTemplate>
            <asp:Label Text='<%# Bind("EBS_x002e_CN_x002e_Inventar_x0028") %>' runat="server"
                ID="lblProduct" />
        </ItemTemplate>
    </asp:TemplateField>
</Columns>

Now you can sort on all 3 columns: