title: Mongoose get document as an array of arrays based on a key
body: <p>I don't really know how to frame the question but what I have is the following schema in mongoose</p>
  
  <pre><code>new Schema({
  
                          gatewayId: { type: String, index: true },
                          timestamp: { type: Date, index: true },
                          curr_property:Number,
                          curr_property_cost:Number,
                          day_property:Number,
                          day_property_cost: Number,
                          curr_solar_generating: Number,
                          curr_solar_export:Number,
                          day_solar_generated:Number,
                          day_solar_export:Number,
                          curr_chan1:Number,
                          curr_chan2:Number,
                          curr_chan3:Number,
                          day_chan1:Number,
                          day_chan2:Number,
                          day_chan3:Number
  },{
      collection: 'owlelecmonitor'
  });
  </code></pre>
  
  <p>and I want to be able to query all the documents in the collection but the data should be arranged inside the array in the following format </p>
  
  <pre><code>[ [{
      gatewayId: 1,
      timestamp: time
      ....
     },
     {
  
      gatewayId: 1,
      timestamp: time2
      ....
     }],
    [{
      gatewayId: 2,
      timestamp: time
      ....
     },
     {
  
      gatewayId: 2,
      timestamp: time2
      ....
  
     }],
    [{
      gatewayId: 3,
      timestamp: time
      ....
  
     },
     {
  
      gatewayId: 3,
      timestamp: time2
      ....
  
     }] 
  ];
  </code></pre>
  
  <p>Is there a way that I can do this in mongoose instead of retrieving the documents and processing them again ?</p>