Project invoicing : sub projects on the same invoice

hello,

I created a project with sub projects linked with an analytic account (view type) and sub analytic account (normal type).

I would like to invoice the (parent) project on a single invoice (including sub projects ).

How can I perform that?

For now I can just select the entries and click the 'invoice analytic lines' link and create an invoice per sub analytic account (sub project).

Thanks for your answer.