[{_id:1,"_a":[{"_aId":{"CC":"CA"},"_p":[{"_pId":{"CC":"CA"},"s":{"c":{"t":[{sId:1,language:"CA",format:"A4"},{sId:2,language:"JP",format:"A4"}]},"a":{"t":[{sId:4,language:"CA","format":"A4"},{sId:5,language:"EN","format":"A3"}]},"d":{"t":[{sId:10,language:"CA","format":"A4"}]}}}]},{"_aId":{"CC":"CAD"},"_p":[{"_pId":{"CC":"CA"},"s":{"c":{"t":[{sId:1,language:"CA",format:"A44"},{sId:2,language:"JP",format:"A44"}]},"a":{"t":[{sId:79,language:"CA","format":"A4"},{sId:5,language:"EN","format":"A3"}]},"d":{"t":[{sId:10,language:"CA","format":"A455"}]}}}]}]}]

db.collection.aggregate([{$project:{_id:0,res:{$reduce:{input:"$_a",initialValue:[],in:{$concatArrays:["$$value",{$reduce:{input:"$$this._p",initialValue:[],in:{$concatArrays:["$$value",["$$this.s"]]}}}]}}}}},{$project:{res:{$map:{input:"$res",in:{$objectToArray:"$$this"}}}}},{$project:{res:{$reduce:{input:"$res",initialValue:[],in:{$concatArrays:["$$value",{$reduce:{input:"$$this",initialValue:[],in:{$concatArrays:["$$value","$$this.v.t"]}}}]}}}}},{$project:{res:{$filter:{input:"$res",cond:{$and:[{$eq:["$$this.language","CA"]},{$eq:["$$this.format","A4"]}]}}}}},{$unwind:"$res"},{$project:{sId:"$res.sId"}}])