multi-currency

Multi-Currency Domain Knowledge

Supported Currencies

enum Currency {
  USD  // US Dollar
  EUR  // Euro
  ILS  // Israeli Shekel
}

Exchange Rate Storage

model ExchangeRate {
  id             String   @id @default(cuid())
  baseCurrency   Currency
  targetCurrency Currency
  rate           Decimal  @db.Decimal(12, 6)  // 6 decimal precision
  date           DateTime
  fetchedAt      DateTime @default(now())

  @@unique([baseCurrency, targetCurrency, date])
}

Frankfurter API Integration

Location: src/lib/currency.ts

Base URL: https://api.frankfurter.dev/v1

fetchExchangeRates

export async function fetchExchangeRates(baseCurrency: Currency): Promise<FrankfurterResponse>

Features:

• Request deduplication via inFlightRequests Map
• Concurrent requests for same base currency return same Promise
• Automatic cleanup after request completes

getExchangeRate

export async function getExchangeRate(from: Currency, to: Currency, date?: Date): Promise<number>

Strategy:

1. Same currency returns 1 (no conversion needed)
2. Check cache (ExchangeRate table) for today's rate
3. Fetch from API if not cached
4. Upsert to cache
5. Fallback to most recent cached rate if API fails

convertAmount

export async function convertAmount(
  amount: number,
  from: Currency,
  to: Currency,
  date?: Date
): Promise<number>

• Returns amount unchanged if same currency
• Rounds to 2 decimal places: Math.round(converted * 100) / 100

Batch Loading Pattern (N+1 Prevention)

For dashboard aggregations, use batch loading:

batchLoadExchangeRates

export async function batchLoadExchangeRates(date?: Date): Promise<RateCache>
// Returns Map<string, number> for O(1) lookups

convertAmountWithCache

export function convertAmountWithCache(
  amount: number,
  from: Currency,
  to: Currency,
  cache: RateCache
): number

• No database calls (uses preloaded cache)
• Logs warning if rate missing, returns original amount
• Use in loops/aggregations to avoid N+1 queries

Cache Key Format

function rateCacheKey(from: Currency, to: Currency): string {
  return `${from}:${to}`  // e.g., "USD:EUR"
}

Refresh Workflow

export async function refreshExchangeRates(): Promise<
  { success: true; updatedAt: Date } | { error: { general: string[] }; updatedAt: Date }
>

• Fetches rates for ALL base currencies
• Upserts all currency pairs for today
• Called via refreshExchangeRatesAction from dashboard UI

User Preference

model User {
  preferredCurrency Currency @default(USD)
}

Dashboard displays amounts in user's preferred currency via conversion.

Key Files

• src/lib/currency.ts - All currency functions
• src/app/actions/misc.ts - refreshExchangeRatesAction
• src/lib/dashboard-ux.ts - Uses batchLoadExchangeRates for aggregation