_id:...
Segment:[{ field: "S1", value: 1 },{ field: "S2", value: 5 },{ field: "Sn", value: 10 },
]

查询方式

db.collection.aggregate( [{ $unwind: "$Segment" },{ $group: {_id: '$_id', sum: { $sum: '$Segment.value' } } } 
] );

结果

{"result" : [{"_id" : ObjectId("51e4772e13573be11ac2ca6f"),"sum" : 16}],"ok" : 1
}

好的教程 https://www.compose.com/articles/aggregations-in-mongodb-by-example/