title: Condition in aggregation mongodb
body: <p>I'm new in Mongodb and I try to learn aggregation.
  My database look like this:</p>
  
  <pre><code>{
    _id : 1,
    a: 5,
    b: 6,
    c : {
      ca: 3
      cb: 5
  }
  },
  {
    _id : 1,
    a: 3,
    b: 4,
    c : {
      ca: 3
      cb: 7
  }
  }
  </code></pre>
  
  <p>Is there anyway to :
  - First: find max in c
  - Second: create a new object like this:</p>
  
  <pre><code>{
     a : [ 5, 3],
     b : [6, 4],
     c : [5, 7]
  }
  </code></pre>
  
  <p>My apologies if this is a repeated question but I wasn't able to find a similar one.</p>