I misunderstood your requirements.
You do not want to find the transactions of a party. You want to fill the transactions array with the field party_title from the party collection? Right?
Please Formatting code and log snippets in posts and publish you sample documents. Include both parties referred in your sample ledger document.
Also publish what you have tried and indicate how it fails so that we do not waste time and propose, like I just did, a solution that does not match your requirements.