$cond统计个数

{ $cond: [ <boolean-expression>, <true-case>, <false-case> ] }
db.activity.insert({"userUrl": "http://news.bbc.co.uk", "userAddress" : "1.2.3.4", "requestTime" : new Date('Jan 01, 2013'), "status": "success"})
db.activity.insert({"userUrl": "http://news.bbc.co.uk", "userAddress" : "1.2.3.4", "requestTime" : new Date('Jan 01, 2013'), "status": "success"})
db.activity.insert({"userUrl": "http://news.bbc.co.uk", "userAddress" : "1.2.3.4", "requestTime" : new Date('Jan 01, 2013'), "status": "error"})
db.activity.insert({"userUrl": "http://news.bbc.co.uk", "userAddress" : "1.2.3.4", "requestTime" : new Date('Jan 01, 2013'), "status": "unavailable"})
db.activity.insert({"userUrl": "http://news.bbc.co.uk", "userAddress" : "1.2.3.4", "requestTime" : new Date('Jan 02, 2013'), "status": "success"})
db.activity.insert({"userUrl": "http://news.bbc.co.uk", "userAddress" : "1.2.3.4", "requestTime" : new Date('Jan 02, 2013'), "status": "error"})
db.activity.insert({"userUrl": "http://news.bbc.co.uk", "userAddress" : "1.2.3.4", "requestTime" : new Date('Jan 02, 2013'), "status": "error"})
db.activity.insert({"userUrl": "http://news.bbc.co.uk", "userAddress" : "1.2.3.4", "requestTime" : new Date('Jan 02, 2013'), "status": "unavailable"})
db.activity.insert({"userUrl": "http://news.bbc.co.uk", "userAddress" : "1.2.3.4", "requestTime" : new Date('Jan 02, 2013'), "status": "unavailable"})

db.activity.aggregate(
    { $group: {
        _id: { date: '$requestTime' },
        success : {
            $sum: { $cond: [ { $eq: [ 'success', '$status' ] } , 1, 0 ] }
        },
        error : {
            $sum: { $cond: [ { $eq: [ 'error', '$status' ] } , 1, 0 ] }
        },
        unavailable : {
            $sum: { $cond: [ { $eq: [ 'unavailable', '$status' ] } , 1, 0 ] }
        }
    } }
);

结果
{
    "result" : [
        {
            "_id" : {
                "date" : ISODate("2013-01-02T00:00:00Z")
            },
            "success" : 1,
            "error" : 2,
            "unavailable" : 2
        },
        {
            "_id" : {
                "date" : ISODate("2013-01-01T00:00:00Z")
            },
            "success" : 2,
            "error" : 1,
            "unavailable" : 1
        }
    ],
    "ok" : 1
}