Database
[{_id:ObjectId("6092d48d96984d233cf77152"),user:"John Doe",movies:[{_id:ObjectId("6092b19345f48a33447468a7"),title:"Alpha",ranking:3},{_id:ObjectId("6092b19345f48a33447468a7"),title:"Bravo",ranking:2},{_id:ObjectId("6092b19345f48a33447468a7"),title:"Charlie",ranking:1}]},{_id:ObjectId("6092d48d96984d233cf77153"),user:"John Doe",movies:[{_id:ObjectId("6092b19345f48a33447468a7"),title:"Alpha",ranking:3},{_id:ObjectId("6092b19345f48a33447468a7"),title:"Bravo",ranking:2},{_id:ObjectId("6092b19345f48a33447468a7"),title:"Charlie",ranking:1}]}]
Query
db.collection.aggregate([{$unwind:"$movies"},{$group:{_id:"$movies.title",ranking:{$sum:"$movies.ranking"}}}])