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

db.collection.aggregate([{"$project":{"solutions":{"$reduce":{"input":"$tasks","initialValue":[],"in":{"$concatArrays":["$$value",{$reduce:{input:"$$this.assigned",initialValue:[],in:{$concatArrays:["$$value",{$filter:{input:"$$this.solutions",cond:{$eq:["$$this._id",1]}}}]}}}]}}}}}])