Database
db={"authors":[{"_id":"61708f71ee435de97b5a82e7","title":"mjGPzpGLKIdea Title","author":"61708f6dee435de97b5a82e2","votes":["6171c8437234d56cfeff9986","6171c8647234d56cfeff99de"]}],"votes":[{"_id":"6171c8437234d56cfeff9986","score":1},{"_id":"6171c8647234d56cfeff99de","score":1}]}
Query
db.authors.aggregate([{"$lookup":{"from":"votes","localField":"votes","foreignField":"_id","as":"scoreSum"}},{"$set":{"scoreSum":{"$reduce":{"input":"$scoreSum","initialValue":0,"in":{"$add":["$$value","$$this.score"]}}},"votes":"$$REMOVE"}}])