UpdateGroupCurrency.ts

// change group currency task registration
export let changeGroupCurrency = functions
    .database 
    .ref('/serverTasks/currencyChange/{taskId}')
    .onWrite(event => changeGroupCurrency(event)); 
    
// function which changes group's currency and updates exchange rates of all expenses
async function changeGroupCurrency(event: Event<DeltaSnapshot>)
{
    // prevent triggering this function after writing response code
    if (event.data.previous.val() != null)
    {
        return;
    }
  
    let root = event.data.ref.root;
    let adminRoot = event.data.adminRef.root;
    let taskId = event.params['taskId'];
    let groupId = event.data.child('request/groupId').val();
    let groupCurrency = event.data.child('request/targetCurrency').val();
    
    try 
    {
        let transactions = (await root.child('transactions/' + groupId).once('value')).val(); 
        let updates = {};

        // iterate over all transactions within given group
        for (let key in transactions) 
        {
            let tx: Transaction = transactions[key];
            let txCurrency = tx.currencyCode;
            
            // if this expense doesn't have an exchange rate for this currency
            if (!tx.exchangeRates || !(groupCurrency in tx.exchangeRates))
            {
                let ts = moment(tx.clientTimestamp);
                let groupCurrencyRate = await getCurrencyRate(root, groupCurrency, ts);
                let txCurrencyRate = await getCurrencyRate(root, txCurrency, ts);
                
                // save the new exchange rate into a single 'updates' object
                updates['transactions/' + groupId + '/' + key + '/' + 'exchangeRates/' + groupCurrency] = (groupCurrencyRate / txCurrencyRate).toString();
            }
        }

        // check if task was deleted in the meantime, meaning user cancelled it
        let task = (await root.child('serverTasks/currencyChange/' + taskId).once('value')).val();
        if (task) 
        {
            // update the exchange rates            
            await root.update(updates);
            
            // update the group currency
            await root.child('groups/' + groupId + '/convertedToCurrency').set(groupCurrency);
            
            // write response code
            await adminRoot.child('serverTasks/currencyChange/' + taskId + '/response/code').set('ok');
        }
        else
        {
            console.log("Task" + taskId + " was cancelled by the user.");
        }