[{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([{$match:{user_id:"u1"}},{$project:{_id:0,requests:{$reduce:{input:"$subscriptions.requests",initialValue:[],in:{$concatArrays:["$$value","$$this"]}}}}}])