From bd7a55c6d373cad3ceb60ef75c50d3d6593cbea3 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Tue, 26 May 2026 16:45:30 +0000
Subject: [PATCH] fix: always fetch exchange rate by expense date

Agent-Logs-Url: https://github.com/ClementLSW/osps/sessions/7f23da57-fd59-4ff9-8e7d-b17be954e069

Co-authored-by: ClementLSW <24468433+ClementLSW@users.noreply.github.com>
---
 src/lib/exchangeRate.js  | 66 +++++++++-------------------------------
 src/pages/AddExpense.jsx | 12 ++++----
 2 files changed, 21 insertions(+), 57 deletions(-)

diff --git a/src/lib/exchangeRate.js b/src/lib/exchangeRate.js
index f980be8..7bab37c 100644
--- a/src/lib/exchangeRate.js
+++ b/src/lib/exchangeRate.js
@@ -1,69 +1,33 @@
 /**
- * Exchange rate utilities — calls /api/exchange-rate (Netlify Function proxy).
- *
- * The proxy calls api.frankfurter.dev server-side, avoiding browser CORS
- * restrictions. Rate convention: 1 unit of `from` = X units of `to`.
- *
- * ECB publish window:
- *   Rates update daily around 16:00 CET (22:00 SGT). During this window
- *   /latest can be unstable. If the call fails without an explicit date,
- *   we retry once with yesterday's date as a fallback.
+ * Exchange rate utilities — fetches from frankfurter.dev.
+ * Always fetches by explicit date so rate reflects when money was spent.
  */
 
+const BASE_URL = 'https://api.frankfurter.dev/v1'
+
 /**
- * Fetch exchange rate.
- * @param {string} from   - Source currency (ISO 4217, e.g. 'MYR')
- * @param {string} to     - Target currency (ISO 4217, e.g. 'SGD')
- * @param {string} [date] - ISO date string (YYYY-MM-DD) for historical rate
- * @returns {Promise<{ from, to, rate, date, fetchedAt, usedFallbackDate? }>}
+ * @param {string} from - Source currency (e.g. 'MYR')
+ * @param {string} to   - Target currency (e.g. 'SGD')
+ * @param {string} date - ISO date YYYY-MM-DD (the expense date)
+ * @returns {Promise<{ from, to, rate, date, fetchedAt }>}
  */
 export async function fetchExchangeRate(from, to, date) {
   if (from === to) {
-    return {
-      from,
-      to,
-      rate: 1,
-      date: date || new Date().toISOString().slice(0, 10),
-      fetchedAt: new Date().toISOString(),
-    }
+    return { from, to, rate: 1, date, fetchedAt: new Date().toISOString() }
   }
 
-  // First attempt
-  try {
-    return await _fetchOnce(from, to, date)
-  } catch (err) {
-    // Only retry /latest — historical dates don't have publish window issues
-    if (date) throw err
-
-    const yesterday = new Date()
-    yesterday.setUTCDate(yesterday.getUTCDate() - 1)
-    const fallbackDate = yesterday.toISOString().slice(0, 10)
-
-    const result = await _fetchOnce(from, to, fallbackDate)
-    return { ...result, usedFallbackDate: true }
-  }
-}
-
-async function _fetchOnce(from, to, date) {
-  const params = new URLSearchParams({ from, to })
-  if (date) params.set('date', date)
-
-  const res = await fetch(`/api/exchange-rate?${params}`)
+  const res = await fetch(`${BASE_URL}/${date}?from=${from}&to=${to}`)
 
   if (!res.ok) {
-    const body = await res.json().catch(() => ({}))
-    const err = new Error(body.error || `Exchange rate fetch failed: ${res.status}`)
+    const err = new Error(`Exchange rate fetch failed: ${res.status}`)
     err.status = res.status
     throw err
   }
 
   const data = await res.json()
+  const rate = data.rates?.[to]
 
-  return {
-    from: data.from,
-    to: data.to,
-    rate: data.rate,
-    date: data.date,
-    fetchedAt: new Date().toISOString(),
-  }
+  if (rate == null) throw new Error(`No rate found for ${from} → ${to}`)
+
+  return { from, to, rate, date: data.date, fetchedAt: new Date().toISOString() }
 }
diff --git a/src/pages/AddExpense.jsx b/src/pages/AddExpense.jsx
index 6345efa..528ec22 100644
--- a/src/pages/AddExpense.jsx
+++ b/src/pages/AddExpense.jsx
@@ -65,33 +65,33 @@ export default function AddExpense() {
   // Fetch exchange rate when currency or date changes
   useEffect(() => {
     const groupCurrency = group?.currency || 'SGD'
-    if (!expenseCurrency || expenseCurrency === groupCurrency || manualRate) return
+    if (!expenseCurrency || expenseCurrency === groupCurrency || manualRate) {
+      setRateLoading(false)
+      return
+    }
 
     let cancelled = false
     async function fetchRate() {
       setRateLoading(true)
       try {
-        const isToday = expenseDate === new Date().toISOString().slice(0, 10)
         const result = await fetchExchangeRate(
           expenseCurrency,
           groupCurrency,
-          isToday ? undefined : expenseDate
+          expenseDate,
         )
         if (!cancelled) setExchangeRate(result)
       } catch (err) {
-        const isHistorical = expenseDate !== new Date().toISOString().slice(0, 10)
         await log('fx.error', {
           from: expenseCurrency,
           to: group?.currency || 'SGD',
           date: expenseDate,
-          is_historical: isHistorical,
           http_status: err.status ?? null,
           error: err.message,
         })
         console.error('Exchange rate fetch failed:', err)
         if (!cancelled) toast.error('Could not fetch exchange rate')
       } finally {
-        if (!cancelled) setRateLoading(false)
+        setRateLoading(false)
       }
     }
     fetchRate()