在mongodb中相乘表示仅对字符串类型进行操作

Question

我试图在mongodb中乘以2个字段。两者均为数字类型，但mongodb返回$multiply only supports numeric types。该集合是：

{
        "_id" : ObjectId("55e07eb54acc499bb3daae6a"),
        "propertytype" : "Hotel",
        "name" : "Rude Lounge2",
        "costing" : {
                "vegperplate" : 350,
                "nonvegperplate" : 450,
                "flatcharge" : 20000
        },
        "capacity" : {
                "min" : 90,
                "max" : 200
        }
}
{
        "_id" : ObjectId("55e07ebe4acc499bb3daae6b"),
        "propertytype" : "Hotel",
        "name" : "Rude Lounge3",
        "costing" : {
                "vegperplate" : 350,
                "nonvegperplate" : 450,
                "flatcharge" : 20000
        },
        "capacity" : {
                "min" : 90,
                "max" : 200
        }
}

我的查询是：

> db.properties2.aggregate(
... { $project: {
...  "cost": {
... $multiply:["costing.vegperplate","capacity.min"]
...       }
... }
... })

错误堆栈为：

assert: command failed: {
        "errmsg" : "exception: $multiply only supports numeric types, not String",
        "code" : 16555,
        "ok" : 0
} : aggregate failed
Error: command failed: {
        "errmsg" : "exception: $multiply only supports numeric types, not String",
        "code" : 16555,
        "ok" : 0
} : aggregate failed
    at Error (<anonymous>)
    at doassert (src/mongo/shell/assert.js:11:14)
    at Function.assert.commandWorked (src/mongo/shell/assert.js:244:5)
    at DBCollection.aggregate (src/mongo/shell/collection.js:1149:12)
    at (shell):1:16
2015-08-29T00:13:39.173-0400 Error: command failed: {
        "errmsg" : "exception: $multiply only supports numeric types, not String",
        "code" : 16555,
        "ok" : 0
} : aggregate failed at src/mongo/shell/assert.js:13

有人在我做错什么吗？

Answer 1

$由于操作是在文档的两个不同字段上进行的，因此需要在字段名称前加上符号。

db.properties2.aggregate([
    { 
        "$project": {
            "cost": { "$multiply": [ "$costing.vegperplate", "$capacity.min" ]}
    }
])