Sortable by count ManyToMany relations

[ArmandArthur]

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?