Cross-currency envelope activity uses month/dynamic FX rate instead of transaction-date rate

[osipxd]

Summary

If a transaction currency differs from the envelope/category currency, amounts are converted using a month-level (dynamic) FX rate, not the rate effective on the transaction date.

Expected

Historical transaction values in envelopes/categories should be converted using transaction-date FX (or another stable historical policy), so past totals don’t change when rates update.

Actual

Past envelope/category activity can be revalued as FX changes.

Code references

• src/5-entities/envBalances/3 - envMetrics.ts
  uses convertFx(selfAvailableRaw, currency, month)
• src/2-pages/Budgets/EnvelopePreview/EnvelopeInfo.tsx
  converts totals with convertFx(a, currency, month)

Impact

Historical envelope/category totals are unstable for transactions with currency different from the category currency. Because of that I never have categories with available = 0.

[osipxd]

It seems that I should freeze exchange rates for each month by clicking "Download exchange rates". I've found this feature only after searching through sources, so this is not a bug, but a feature observability problem.

[ardov]

Yep, definitely will make it more obvious 😅

[osipxd]

I've drafted a feature freezing exchange rates for past months during sync. On my account with 5-year history it worked good. I can open a draft PR into beta or main branch if this change is acceptable.

[ardov]

I'd hold it for a while. For regular use the past is not that interesting, so I think the main value for rates is to pin them. Even one set of frozen rates does it.
In future I'd want to make some kind of onboarding or a set of recommendations and one of them will be to freeze rates.