[{"categoryId":"1234","sellerId":"2323","productId":"121","rating":1},{"categoryId":"1234","sellerId":"2323","productId":"121","rating":-1},{"categoryId":"1235","sellerId":"2323","productId":"122","rating":-1},{"categoryId":"1234","sellerId":"2323","productId":"123","rating":-1},{"categoryId":"1235","sellerId":"2323","productId":"124","rating":1},{"categoryId":"1234","sellerId":"2323","productId":"125","rating":1},{"categoryId":"1234","sellerId":"2323","productId":"125","rating":1}]

db.collection.aggregate([{"$match":{"sellerId":"2323"}},{"$group":{"_id":"$categoryId","positiveRatingCount":{"$sum":{"$cond":[{"$gt":["$rating",0]},"$rating",0]}},"negativeRatingCount":{"$sum":{"$cond":[{"$lt":["$rating",0]},"$rating",0]}}}}])