[{"_id":1,"owner":"Alice",airline:"RSAirlines","content":["shoes","pants","sockets"]},{"_id":2,"owner":"Bob",airline:"RSAirlines","content":["phone","pants"]},{"_id":3,"owner":"Charlie",airline:"RSAirlines","content":["shoes","pants","bag"]},{"_id":4,"owner":"Mary",airline:"AirES","content":["sandals","coins","sockets"]},{"_id":5,"owner":"Olivia",airline:"AirES","content":["gloves","souvenir","sockets"]},{"_id":6,"owner":"Dan",airline:"AirES","content":["sockets","wallet"]},{"_id":7,"owner":"Erin",airline:"AirES","content":["pants","sockets","dress"]},]

db.collection.aggregate([{$unwind:"$content"},{$group:{_id:{airline:"$airline",content:"$content"},count:{$sum:1}}},{$group:{_id:"$_id.airline",counts:{$push:{k:"$_id.content",v:"$count"}}}},{$project:{counts:{$arrayToObject:"$counts"}}}])