title: Aggregate group multiple fields
body: <p>Given the following dataset:</p>
  
  <pre><code>{ "_id" : 1, "city" : "Yuma", "cat": "roads", "Q1" : 0, "Q2" : 25, "Q3" : 0, "Q4" : 0 }
  { "_id" : 2, "city" : "Reno", "cat": "roads", "Q1" : 30, "Q2" : 0, "Q3" : 0, "Q4" : 60 }
  { "_id" : 3, "city" : "Yuma", "cat": "parks", "Q1" : 0, "Q2" : 0, "Q3" : 45, "Q4" : 0 }
  { "_id" : 4, "city" : "Reno", "cat": "parks", "Q1" : 35, "Q2" : 0, "Q3" : 0, "Q4" : 0 }
  { "_id" : 5, "city" : "Yuma", "cat": "roads", "Q1" : 0, "Q2" : 15, "Q3" : 0, "Q4" : 20 }
  </code></pre>
  
  <p>I'm trying to achieve the following result. It would be great to just return the totals greater than zero, and also compress each city, cat and Qx total to a single record.</p>
  
  <pre><code>{
      "city" : "Yuma",
      "cat" : "roads",
      "Q2total" : 40
  }, 
  {
      "city" : "Reno",
      "cat" : "roads",
      "Q1total" : 30
  },
  {
      "city" : "Reno",
      "cat" : "roads",
      "Q4total" : 60
  },
  {
      "city" : "Yuma",
      "cat" : "parks",
      "Q3total" : 45
  },
  {
      "city" : "Reno",
      "cat" : "parks",
      "Q1total" : 35
  },
  {
      "city" : "Yuma",
      "cat" : "roads",
      "Q4total" : 20
  }
  </code></pre>
  
  <p>Possible?</p>