MongoDB 聚合错误:管道阶段规范对象必须恰好包含一个字段

问题描述

我是 mongodb 的新手，第一次尝试聚合.在这里，我试图获取每 15 分钟分组的推文计数.当我尝试在 mongo 控制台中运行以下查询时，出现错误:

I am new to mongodb and trying out aggregation for first time. Here, I am trying to get the count of tweets grouped by every 15 minutes. When I try to run the below query in mongo console I get the error:

管道阶段规范对象必须只包含一个字段.

    db.hashtag.aggregate([
    { "$group": {
        "_id": {
            "year": { "$year": "$tweettime" },
            "dayOfYear": { "$dayOfYear": "$tweettime" },
            "interval": {
                "$subtract": [ 
                    { "$minute": "$tweettime" },
                    { "$mod": [{ "$minute": "$tweettime"}, 15] }
                ]
            }
        }},
        "count": { "$sum": 1 }
    }
])

我在 SO 中找不到对原因的很好解释.请分享您对此主题的看法以及为什么我的查询出错.

I couldn't find a good explanation of the reason in SO. Kindly share your thoughts on this subject and why my the query has an error.

推荐答案

MongoDB 正在抱怨，因为您的管道中有一个无法识别的管道阶段规范 "count": { "$sum": 1 }.

MongoDB is complaining because you have an unrecognised pipeline stage specification "count": { "$sum": 1 } in your pipeline.

正确格式化后的原始管道

Your original pipeline when formatted properly

db.hashtag.aggregate([
    { 
        "$group": {
            "_id": {
                "year": { "$year": "$tweettime" },
                "dayOfYear": { "$dayOfYear": "$tweettime" },
                "interval": {
                    "$subtract": [ 
                        { "$minute": "$tweettime" },
                        { "$mod": [{ "$minute": "$tweettime"}, 15] }
                    ]
                }
            }
        },
        "count": { "$sum": 1 } /* unrecognised pipeline specification here */
    }
])

应该有聚合累加器$sum 在 $group 管道为:

should have the aggregate accumulator $sum within the $group pipeline as:

    { 
        "$group": {
            "_id": {
                "year": { "$year": "$tweettime" },
                "dayOfYear": { "$dayOfYear": "$tweettime" },
                "interval": {
                    "$subtract": [ 
                        { "$minute": "$tweettime" },
                        { "$mod": [{ "$minute": "$tweettime"}, 15] }
                    ]
                }
            },
            "count": { "$sum": 1 }
        }           
    }
])

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