exchange rate data source supports requesting multi url

pkg/models/exchange_rate.go

@@ -1,12 +1,14 @@
 package models
 
+import "strings"
+
 // LatestExchangeRateResponse returns a view-object which contains latest exchange rate
 type LatestExchangeRateResponse struct {
-	DataSource    string                `json:"dataSource"`
-	ReferenceUrl  string                `json:"referenceUrl"`
-	UpdateTime    int64                 `json:"updateTime"`
-	BaseCurrency  string                `json:"baseCurrency"`
-	ExchangeRates []*LatestExchangeRate `json:"exchangeRates"`
+	DataSource    string                  `json:"dataSource"`
+	ReferenceUrl  string                  `json:"referenceUrl"`
+	UpdateTime    int64                   `json:"updateTime"`
+	BaseCurrency  string                  `json:"baseCurrency"`
+	ExchangeRates LatestExchangeRateSlice `json:"exchangeRates"`
 }
 
 // LatestExchangeRate represents a data pair of currency and exchange rate
@@ -14,3 +16,21 @@ type LatestExchangeRate struct {
 	Currency string `json:"currency"`
 	Rate     string `json:"rate"`
 }
+
+// LatestExchangeRateSlice represents the slice data structure of LatestExchangeRate
+type LatestExchangeRateSlice []*LatestExchangeRate
+
+// Len returns the count of items
+func (c LatestExchangeRateSlice) Len() int {
+	return len(c)
+}
+
+// Swap swaps two items
+func (c LatestExchangeRateSlice) Swap(i, j int) {
+	c[i], c[j] = c[j], c[i]
+}
+
+// Less reports whether the first item is less than the second one
+func (c LatestExchangeRateSlice) Less(i, j int) bool {
+	return strings.Compare(c[i].Currency, c[j].Currency) < 0
+}