class Party(metaclass=PoolMeta):
    __name__ = 'party.party'
    currency = fields.Many2One('currency.currency',
            'Currency', states=STATES, depends=DEPENDS)

class Purchase(metaclass=PoolMeta):
    __name__ = 'purchase.purchase'

    def on_change_party(self):
        super().on_change_party()

        if self.party and self.party.currency:
            self.currency = self.party.currency
        else:
            Currency = Pool().get('currency.currency')
            self.currency = Currency(self.default_currency())


class PurchaseRequisition(metaclass=PoolMeta):
    __name__ = 'purchase.requisition'

    def on_change_supplier(self):
        super().on_change_supplier()

        if self.supplier and self.supplier.currency:
            self.currency = self.supplier.currency
        else:
            Currency = Pool().get('currency.currency')
            self.currency = Currency(self.default_currency())

class Sale(metaclass=PoolMeta):
    __name__ = 'sale.sale'

    def on_change_party(self):
        super().on_change_party()

        if self.party and self.party.currency:
            self.currency = self.party.currency
        else:
            Currency = Pool().get('currency.currency')
            self.currency = Currency(self.default_currency())