I declared a  computed field:

'member_still_pay': fields.float(compute='_get_total', type='float', string='Still to pay')

@api.multi

def _get_total(self):

temp = 0.0

for rec in self:

temp = sum(line.payment_amount for line in rec.member_payments)

rec.member_still_pay = rec.member_cotisation - temp

The problem is I don't have the good value in member_still_pay. The field payment_amount is a one2many. Somebody know what is false?