How to bind WPF Datagrid radiobutton column to entities class?

[1 Sparrow]

Good morning. I'm Eugene Buzin from Russia.
In my WPF application I created a LINQ to SQL entities class. Then I created a partial class and there add code to my entity class. Below is the code of it:

namespace Mynamespace
{
    partial class rts_index
    {
        #region Fields

        /// <summary>
        /// Status:
        /// "Selected" - true, "Unselected" - false.
        /// </summary>
        private bool _is_selected;

        #endregion

        #region Properties

        /// <summary>
        /// Get or set the status:
        /// "Selected" - true, "Unselected" - false.
        /// </summary>
        public bool is_selected
        {
            get { return this._is_selected; }
            set { this._is_selected = value; }
        }

        #endregion
    }
}

Then I executed LINQ to SQL. Works perfectly well. Then I bound WPF Datagrid radiobutton column to is_selected property of the partial class. Below is XAML code:

.
.
.
<Window.Resources>
<CollectionViewSource x:Key="rts_indexViewSource" d:DesignSource="{d:DesignInstance my:rts_index, CreateList=True}" />
</Window.Resources>
.
.
.
<DataGrid Name="dgStockMarketIndex" Grid.Column="1"  Grid.Row="1" AutoGenerateColumns="False"
                  Margin="10" SelectionMode="Extended" CanUserAddRows="False" CanUserDeleteRows="False"
                  CanUserReorderColumns="False" CanUserSortColumns="False" CanUserResizeRows="False"
                  ItemsSource="{Binding Source={StaticResource rts_indexViewSource}}">
            <DataGrid.Columns>
                <DataGridTemplateColumn Header="Selected">
                    <DataGridTemplateColumn.CellTemplate>
                        <DataTemplate>
                            <RadioButton GroupName="IndexSelection"
                                         IsChecked="{Binding Path=is_selected, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}"/>
                        </DataTemplate>
                    </DataGridTemplateColumn.CellTemplate>
                </DataGridTemplateColumn
.
.
.

And here I have the problem. When I'm changing the radiobutton status from Unchecked to Checked it doesn't change the value of is_selected property from false to true in the partial class. I don't know why. Please help me if you can.

Yours sincerely Eugene Buzin.

[blackflys]

I have exactly the same problem, has somebody found an answer to this? The radio button keeps unchecking when I focus on another line of my datagrid.