title: Mongo Aggregate Sum of values in a list of dictionaries for all documents
body: <p>I have a collection 'comments' as follows:</p>
  
  <pre><code>{
  comment_id:10001
  aspects:[
   {
     name:'aspectA',
     positive:2
     negative:3
     neutral:1
    },
   {
     name:'aspectB',
     positive:1
     negative:5
     neutral:3
    }
  },
  {
  comment_id:10002
  aspects:
   {
     name:'aspectA',
     positive:2
     negative:1
     neutral:2
    },
   {
     name:'aspectB',
     positive:3
     negative:4
     neutral:1
    }
  }
  ]
  </code></pre>
  
  <p>the count of documents in comments is greater than 100K. I have to find the count of positive, negative and neutral for all aspects, ie. the sum of positive, negative and neutral for each aspect from the <code>aspects</code> field (which is a list of dicts as mentioned above) for all documents. I found that mapreduce can be used to do the task but I couldn't find enough docs to build a query. </p>
  
  <p>Is there a way to find this using one query?</p>