title: mongodb - want to sum values over time
body: <p>I have a bunch of documents in Mongo that represent sales transactions - 1 document per transaction. Within each document, I have the value of the transaction and the date&amp;time. I want to run a query that sums up the total value of all transactions for each day.</p>
  
  <p>I have looked at the aggregation framework, but I can't find any examples of using a timestamp/date to do the filtering. Can anyone give me some pointers on how to do this?</p>