(Or files, that would also be useful.)

Closely related to existing feature request: https://gnucash.uservoice.com/forums/101223-feature-request/suggestions/1951947-user-defined-fields-attributes

I often have a single transaction represented in multiple systems. For example, say I pay a grocery order which is split between me and my flatmates.

• The grocery order has an "order number" associated with the merchant (a string like abc-12345-xyz).
• The order also has a URL with the merchant.
• We use Splitwise to split transactions, so I'd also like to attach the Splitwise URL to the transaction. (Basically if I pay $250 for groceries, there's a maybe $50 split that goes to "Expenses:Groceries", then maybe $200 that goes to "Assets:My flatmates owe me in Splitwise".)
• We also have a Google Sheet in which we fill out which portion of which item does each flatmate pay for. It would be also nice to link the particular order's sheet to the transaction.

Unfortunately right now GnuCash only seems to allow connecting one URL per transaction.

Ideally I'd like it if I could save multiple URLs, and with each URL (or other identifier), declare "this is the URL for the spreadsheet", "this is the link to Splitwise", "this is the order ID in the merchant", "this is my bank's ID" etc.

If I could declare these "attachment types" and manage them, I could then also query the GnuCash database to semi-automatically check the mapping between Splitwise, GnuCash etc. - "for every transaction that has a URL with type 'Splitwise transaction', fetch the transaction from Splitwise API and check that my personal share is the part that's in my expenses".