わかりました。mongodb でこれができるかどうかはわかりませんが、次の JSON を通貨 DB に挿入する必要があります。
更新したい部分は exchangehistory で、その日の為替レートのすべての履歴を保持する必要があります。そして翌日、例えば
例えば
{"from":"USD","currentexchange":[{"to":"NZD","rate":"1.3194","updated":"6\/5\/20121:38am"},{"to":"KWD","rate":"0.2807","updated":"6\/5\/20121:38am"},{"to":"GBP","rate":"0.6495","updated":"6\/5\/20121:38am"},{"to":"AUD","rate":"1.0228","updated":"6\/5\/20121:38am"}],"exchangehistory":{"6\/5\/2012":[{"1:38am":[{"to":"NZD","rate":"1.3194","updated":"1:38am"}]},{"1:38am":[{"to":"KWD","rate":"0.2807","updated":"1:38am"}]},{"1:38am":[{"to":"GBP","rate":"0.6495","updated":"1:38am"}]},{"1:38am":[{"to":"AUD","rate":"1.0228","updated":"1:38am"}]}]}}