popup combobox on pressing enter in a GtkComboboxEntry

hi.
how can i do this? 

it seems that :

<?php
$shopLogCombobox = $glade->get_widget('shopLogCombobox');
$shopLogCombobox->connect_simple('editing-done', 'test');
?>

or 

<?php
$shopLogCombobox = $glade->get_widget('shopLogCombobox');
$shopLogCombobox->get_child()->connect_simple('editing-done', 'test');
?>

does not work.

thx