Sortable by count ManyToMany relations

I have a command's list relate to many products.
I display in a column the number of products.

How sort on this number?

`            <td {% if commandes.isSorted('count(r.commandProducts)') %} class="sorted"{% endif %}>
                    {{ knp_pagination_sortable(commands, 'Number products', 'count(r.commandProducts)') }}
            </td>`

Or there exist an other way?
