[{_id:ObjectId("62015250cb18d3cadb32c38e"),user:{pk:"7485070525"},usertags:[{user:{pk:"123456"}}]},{_id:ObjectId("62015250cb18d3cadb32c38f"),user:{pk:"334777488"},usertags:[{user:{pk:"7485070525"}},{user:{pk:"271738610"}},{user:{pk:"31961021"}}]}]

db.collection.aggregate([{$group:{_id:null,user_pk:{$push:"$user.pk"},user_sub_pk:{$push:"$usertags"}},},{$set:{user_sub_pk:{$reduce:{input:"$user_sub_pk",initialValue:[],in:{$concatArrays:["$$value","$$this"],},},},},},{$project:{user_pks:{$concatArrays:["$user_pk","$user_sub_pk.user.pk"],},},},{"$project":{"user_pks":{"$setUnion":["$user_pks",[]]}}}])