Storing exchange rate and currency on transaction level

[jakubkottnauer]

Currently in Maybe, there is the base family currency and the currency of each individual account. I'd say it's worth extending the model to be even more granular, that is to allow transactions themselves to have a different currency (+ exchange rate which would be 1.0 when the currency of the transaction and account are the same).

For example, this is how my credit card (in SEK) reports transactions done in a foreign currency (CZK in this example) when queried via GoCardless:

{
    "transactionId": "--",
    "entryReference": "--",
    "bookingDate": "2024-01-24",
    "valueDate": "2024-01-25",
    "bookingDateTime": "2024-01-24T00:00:00+00:00",
    "valueDateTime": "2024-01-25T00:00:00+00:00",
    "transactionAmount": {
        "amount": "-2439.19",
        "currency": "SEK"
    },
    "currencyExchange": {
        "sourceCurrency": "SEK",
        "exchangeRate": "2.1728",
        "targetCurrency": "CZK"
    },
    "creditorName": "pub",
    "remittanceInformationUnstructured": "",
    "additionalInformation": "--",
    "internalTransactionId": "--"
}

Without this, this extra information of the imported transaction would get lost.

A situation where this would be helpful is when you travel and enter cash transactions manually in a currency you only use during the vacation. Instead of having to create a new account just for that currency, you could reuse your usual "cash" account and enter values in the different currency, having Maybe suggest the current exchange rate.

Here's a screenshot from a competing app that does his quite nicely:

[zachgoll]

@jakubkottnauer while the implementation has not been completed for this yet, we are storing transaction level currency here:

maybe/db/schema.rb

Lines 203 to 204 in 159245b

	t.decimal "amount", precision: 19, scale: 4, null: false
	t.string "currency", default: "USD", null: false

I think similar to how you show in the data payload from GoCardless, it might be a good idea to store a "cached" value on the transaction that stores something like family_amount and family_currency, representing the converted values based on exchange rates.

[zachgoll]

Implemented