I have a relation between the fuel and voucher classes with the field 'amount_used' that calculates the number of voucher x used in fuel without problems 

_name = 'fleet.vehicle.log.fuel'
'voucher_id': fields.many2one('fleet.voucher', 'Voucher'), 

_name = 'fleet.voucher' 
'amount_used': fields.function(_count_all, type='integer', string="Amount Used") 

def _count_all(self, cr, uid, ids, field_name, arg, context=None):
        Fuel = self.pool['fleet.vehicle.log.fuel']
        return {
 voucher_id: Fuel.search_count(cr, uid, [('voucher_id', '=', voucher_id)], context=context) 
 for voucher_id in ids 
        }

when I add the store parameter in 'amount_used' I have an update problem

store=True or store={'fleet.vehicle.log.fuel': (lambda self, cr, uid, ids, c={}: ids, ['voucher_id'], 10)}