assert self.value_from.currency == self.value_to.currency
assert self.value_from.linked_to is not None and self.value_from.linked_to.currency == self.currency
self.base_currency = self.value_from.currency
assert self.value_from.currency == self.value_to.currency
assert self.value_from.linked_to is not None and self.value_from.linked_to.currency == self.currency
self.base_currency = self.value_from.currency