Mogodb Cookbook

gistfile1.txt

// to prevent empty objects being pushed in resule like 'eventWinners' : [ {} ]
// if there are no winners found, after lookup and unwind.

$group: {
              _id: '$_id',
              root: {
                $mergeObjects: '$$ROOT',
              },
              eventWinners: {
                $push: {
                  $cond: {
                    if: { $eq: ["$eventWinners", {}] },
                    then: '$$REMOVE',
                    else: '$eventWinners',
                  }
                }
              },
            },