title: How to aggregate and merge the result into a collection?
body: <p>I want to aggregate and insert the results into an existing collection, without deleting that collection. The documentation seems to suggest that this isn't directly possible. I find that hard to believe.</p>
  
  <p>The map-reduce functionality has 'output modes', including 'merge', which does what I want. I'm looking for the equivalent for aggregation. </p>
  
  <p>The new <a href="http://docs.mongodb.org/master/reference/operator/aggregation/out/" rel="noreferrer"><code>$out</code></a> aggregation stage supports inserting into a collection, but it replaces the collection rather than updating it. If I did this I would (I think) have to run another map-reduce to merge this into another collection, which seems inefficient. </p>
  
  <p>Am I missing something or is the functionality just missing from the aggregation feature?</p>