[{userName:"jdoe",uid:12343,contacts:[{name:"Jane Doe",tasks:[{taskName:"Task1"},{taskName:"Task2"},{taskName:"Task3"},]},{name:"Bob Smith",tasks:[{taskName:"TaskA"},{taskName:"TaskB"},{taskName:"TaskC"},]}]}]

db.collection.aggregate([{$project:{"tasks":{$reduce:{input:"$contacts",initialValue:[],in:{$concatArrays:["$$value","$$this.tasks"]}}},_id:0}}])