[{"client_id":"abc","product_id":"123","weight":{"value":100,"unit":"kg"}},{"client_id":"def","product_id":"456","weight":{"value":200,"unit":"kg"}},{"client_id":"abc","product_id":"123","weight":{"value":1,"unit":"kg"}}]

db.collection.aggregate([{"$match":{"client_id":"abc","product_id":"123"}},{"$group":{"_id":{"client_id":"$client_id","product_id":"$product_id"},"result":{"$sum":"$weight.value"}}},{"$project":{"result":1}}])