[{user_id:"u1",subscriptions:[{"tier":"Basic","requests":[{"name":"aaaa"},{"name":"bbbb"}]},{"tier":"Premium","requests":[{"name":"cccc"},{"name":"dddd"}]}]},{user_id:"u2",subscriptions:[{"tier":"Premium","requests":[{"name":"eeee"},{"name":"ffff"}]}]}]

db.collection.aggregate([{$match:{user_id:"u1"}},{$unwind:"$subscriptions"},{$unwind:"$subscriptions.requests"},{$project:{"name":"$subscriptions.requests.name",_id:0}}])