title: mongoose sum a value across all documents
body: <p>I am looking to count all columns with the name amount in the documents that match my query,</p>
  
  <pre><code> tickets.count({time: {$gte: a}, time: {$lte: tomorrow}}).then(function (numTickets) {
  </code></pre>
  
  <p>How can I get the total result of the document column called amount?</p>
  
  <p>Example, if I do have:</p>
  
  <pre><code>{ time: 20, amount: 40}
  { time: 40, amount: 20}
  </code></pre>
  
  <p>it would return the total amount(60)?</p>
  
  <p>Remember that I do need to use <code>{time: {$gte: a}, time: {$lte: tomorrow}</code> in the query.</p>
  
  <p>How would I do this?</p>