Mongoose group by parent

gistfile1.js

function doSumByGroup() {
  var agg = [
    {$group: {
      _id: "$columnObject",
      total: {$sum: 1}
    }}
  ];
 
  Snippet.aggregate(agg, function(err, logs){
    if (err) { 
      handleErrors();
    } else {
      console.log(logs);
    }
  });
}