class Amendment(metaclass=PoolMeta):
    __name__ = 'sale.amendment'

    customer = fields.Function(fields.Many2One('party.party', 'Customer',
        select=True), 'on_change_with_customer', searcher='search_customer')

    @fields.depends('sale')
    def on_change_with_customer(self, name=None):
        if self.sale:
            return self.sale.party.id

    @classmethod
    def search_customer(cls, name, clause):
        return [('sale.party',) + tuple(clause[1:])]