title: MongoDB $project: Retain previous pipeline fields
body: <p>Is there a way, in a MongoDB projection, to specify some new fields (but at the same time, retain fields that were input to the projection stage of the pipeline)? I'm not renaming any of the existing fields.</p>
  
  <p>So if I start with a collection that has 'field1' and 'field2', and do the following projection:</p>
  
  <pre><code>{ $project: { field3: { $gt: ['$field1', 10] } } }
  </code></pre>
  
  <p>I want to end up with 'field1', 'field2', and 'field3' present as input to the next stage, or output from the aggregation framework.</p>
  
  <p>I attempted to put the projection into exclusion mode by excluding _id, but that doesn't work.</p>