MongoDB 聚合 $divide 计算字段

我试图根据计算字段计算MongoDB查询中的百分比——不确定这是否可行.我想做的是计算失败率:(failed count / total) * 100

以下是一些示例文档:

    {
            "_id" : ObjectId("52dda5afe4b0a491abb5407f"),
            "type" : "build",
            "time" : ISODate("2014-01-20T22:39:43.880Z"),
            "data" : {
                    "buildNumber" : 30,
                    "buildResult" : "SUCCESS"
            }
    },
    {
            "_id" : ObjectId("52dd9fede4b0a491abb5407a"),
            "type" : "build",
            "time" : ISODate("2014-01-20T22:15:07.901Z"),
            "data" : {
                    "buildNumber" : 4,
                    "buildResult" : "FAILURE"
            }
    },
    {
            "_id" : ObjectId("52dda153e4b0a491abb5407b"),
            "type" : "build",
            "time" : ISODate("2014-01-20T22:21:07.790Z"),
            "data" : {
                    "buildNumber" : 118,
                    "buildResult" : "SUCCESS"
            }
    }

以下是我试图处理的问题.问题出现在FailPercent/$divide行中:

db.col.aggregate([    
    { $match: { "data.buildResult" : { $ne : null } } },
    { $group: {          
        _id: {              
            month: { $month: "$time" },             
            day: { $dayOfMonth: "$time" },             
            year: { $year: "$time" },                       
        },         
        Aborted: { $sum: { $cond :  [{ $eq : ["$data.buildResult", "ABORTED"]}, 1, 0]} },
        Failure: { $sum: { $cond :  [{ $eq : ["$data.buildResult", "FAILURE"]}, 1, 0]} },
        Unstable: { $sum: { $cond : [{ $eq : ["$data.buildResult", "UNSTABLE"]}, 1, 0]} },
        Success: { $sum: { $cond :  [{ $eq : ["$data.buildResult", "SUCCESS"]}, 1, 0]} },
        Total: { $sum: 1 },
        FailPercent: { $divide: [ "Failure", "Total" ] }
    } },     
    { $sort: { "_id.year": 1, "_id.month": 1, "_id.day": 1 } } 
])