[{_id:1,tasks:[{_id:1,assigned:[{_id:1,name:"assigned1",solutions:[{_id:1,name:"solution 1",},{_id:2,name:"solution 2",},],},{_id:2,name:"assigned2",solutions:[{_id:1,name:"solution 1",},{_id:2,name:"solution 2",},],},],},],}]

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