some fields readonly only if existing row

[markalves]

Hi I would like to set some fields readonly only if editing existing row:

Existing rows I want to make readonly have a colum set to 9, I made in this way:

onBeforeEdit: function(index,row){
            var col = $(this).edatagrid('getColumnOption', 'qta');
            alert(getSelectedValue('#dg3', 3))
            if(   getSelectedValue('#dg3', 3) == 9) {
               col.editor = null;
            } else {
                col.editor = 'text';
            }
            }

It works but if after editing these rows then I add a new row selectedvalue remain to 9 and it remain readonly.
Is there a way to verify if new row or after editing clear buffer ?