如何动态设置照明ui：inputSelect默认值？

Question

<ui:inputSelect value="{!v.quotation.Currency__c}">
  <aura:iteration items="{!v.currencyList}" var="crr">
    <ui:inputSelectOption text="{!crr}" label="{!crr}" value="{!v.quotation.Currency__c == crr}"/>
  </aura:iteration>
</ui:inputSelect>

v.quotation.Currency__c == crr无效，为什么？