Add combocbox to radGridView column?

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radgrid

hey guys,
i have a radgridview in a winform project.
the radgridview load data using datasource named servRet like this:

if (ServiceUtil.IsValid(servRet))
            {
                grdNiyar.SetDataSourceByServerReturn(servRet);
            }

now, in column 4 i need the column to be combobox type and load the items from another datasource named servRet1. the code look like that:

BindingSource bindingSource1 = new BindingSource();
bindingSource1.DataSource = servRet1.Value;
GridViewComboBoxColumn coinsColumn = this.grdNiyar.Columns["column1"] as GridViewComboBoxColumn;
coinsColumn.Name = "colMatbeaBabank2";
coinsColumn.HeaderText = "check";
coinsColumn.DataSource = servRet1;
coinsColumn.ValueMember = "KodMatbea";
coinsColumn.DisplayMember = "KodAndShemMatbea";
coinsColumn.FilteringMode = GridViewFilteringMode.ValueMember;
coinsColumn.FieldName = "KodAndShemMatbea";
coinsColumn.Width = 200;
this.grdNiyar.Columns.Add(coinsColumn);

the result i'm getting is nothing!
the column dosent load and it stays empty but in run time i can see that the datasource loaded correctly.
any advice?