1. MongoDb combine $facet results into a single result set
    db.getCollection('list').aggregate([
        {
            $facet: {
                "events":[{
                    $match: {
                       'type': 'Event'
                    }
                }],
                "tasks": [{
                    $match: {
                       'type': 'Task'
                    }
                }]
            }
        },
        {$project: {activity:{$setUnion:['$events','$tasks']}}},
        {$unwind: '$activity'},
        {$replaceRoot: { newRoot: "$activity" }}
    ]);