Slection field updated with calculated options in child table

Hello everybody
after a while again stucked in code… I read everything published for Child tables in DocType but have still problem to refresh Selection field in child table after populating it with calculation result. If I change the field type to Text the values are populated and displayed, but when the field type is Selection I am not able to populate the data in …

        frappe.model.set_value(cdt, cdn, "medzera", medzera);

Then I take the value of medzera field to show it in console where it shows correctly

        var medz = frappe.model.get_value(cdt, cdn, "medzera");
        console.log("ITEM:" + medz);

but selection box does not show values…

I tried also

refresh_field("medzera"); cur_frm.refresh_field("medzera");

also when I try to save the form I got message “… field will get truncated, as max characters allowed is 140” - so the values are there somewhere behind but not shown in selection box…

This try was also not successfull:

cur_frm.get_field("cb_stitky").grid.grid_rows[0].grid_form.field_dict.medzera.df.options = medzera;

What am I doing wrong.