Mongo

gistfile1.json

{
  "transactions": [
    {
      "amount": "10.00",
      "authorization": null,
      "created_at": "2012-08-24T14:02:50-04:00",
      "gateway": "bogus",
      "id": 999225661,
      "kind": "capture",
      "order_id": 450789469,
      "status": "success",
      "test": true,
      "receipt": {}
    }
  ]
}

{
  "transactions": [
    {
      "amount": "10.00",
      "authorization": null,
      "created_at": "2012-08-24T14:02:50-04:00",
      "gateway": "bogus",
      "id": 999225661,
      "kind": "sale",
      "order_id": 450789469,
      "status": "success",
      "test": true,
      "receipt": {}
    }
  ]
}

You should be able to do this. You must initialize that object property before you can work on it. If the transactions are coming in as an array of objects then you will have to loop through like so

    order.transaction = [];

    for (i = 0; i < order.transactions.length; i++) {
        var value = order.transactions[i];
        if (value.kind === 'sale' || value.kind === 'capture' && value.kind === 'success') {
            order.transaction[i] = {
                "created_at": value.created_at
            }
        }
    }

    console.log(order);
```javascript