I let a 'c' on the if condition, unfortunately, I haven't enough karma to edit my own post!...

I completely agree with this method, I tried it and it works! Thank you.

Thanks for this tip. It still work in Odoo 8.0

By the way I fixed the extra 'c'

Not too late at all. Thanks. Still works on Odoo 9.0.

Ok after struggling with another issue on the pivot view, I figured out this hack is causing problem... seems like the pivot view is confused on the groups and javascript is crashing when selecting measures or "Expand all". I think the js code can be fixed to fully support the hack here and I will fill a ticket at Odoo.

There is no issue with the pivot finally!!! Problem is that last line should be:

return super(ReportDetailInvoice, self).read_group(cr, uid, domain, fields, groupby, offset, limit, context, orderby, lazy).

tks, it works, but I must change the first and last line

first line : ... , lazy=True) :

last line : ... , lazy=lazy)

Only for your information, this solution does in Odoo 15 exactly the opposite. It keeps the sum value in the header from the grouped element and removes the value of the field from all child records of the group.

For Odoo 15 is the correct way to add the option group_operator=False to the python declaration of the field.