Mongodb 聚合时如何添加附加信息？

Question

我是初学者，我为我的管道写了一行，但我想在我的输出中添加其他信息，比如屏幕名称，或者 tweets.I 试图在 $group 下添加的数量但是每次都给我一个语法错误

这是我的管道：

def make_pipeline():
    # complete the aggregation pipeline
    pipeline = [
    {
        '$match': { 
            "user.statuses_count": {"$gt":99 },
            "user.time_zone": "Brasilia"
        }
    },
    {
        "$group": {
            "_id": "$user.id",
            "followers": { "$max": "$user.followers_count" }

        }
    },
    {
        "$sort": { "followers": -1 }
    },
    {
        "$limit" : 1
    }
];

我在这个例子中使用它：

{
    "_id" : ObjectId("5304e2e3cc9e684aa98bef97"),
    "text" : "First week of school is over :P",
    "in_reply_to_status_id" : null,
    "retweet_count" : null,
    "contributors" : null,
    "created_at" : "Thu Sep 02 18:11:25 +0000 2010",
    "geo" : null,
    "source" : "web",
    "coordinates" : null,
    "in_reply_to_screen_name" : null,
    "truncated" : false,
    "entities" : {
        "user_mentions" : [ ],
        "urls" : [ ],
        "hashtags" : [ ]
    },
    "retweeted" : false,
    "place" : null,
    "user" : {
        "friends_count" : 145,
        "profile_sidebar_fill_color" : "E5507E",
        "location" : "Ireland :)",
        "verified" : false,
        "follow_request_sent" : null,
        "favourites_count" : 1,
        "profile_sidebar_border_color" : "CC3366",
        "profile_image_url" : "http://a1.twimg.com/profile_images/1107778717/phpkHoxzmAM_normal.jpg",
        "geo_enabled" : false,
        "created_at" : "Sun May 03 19:51:04 +0000 2009",
        "description" : "",
        "time_zone" : null,
        "url" : null,
        "screen_name" : "Catherinemull",
        "notifications" : null,
        "profile_background_color" : "FF6699",
        "listed_count" : 77,
        "lang" : "en",
        "profile_background_image_url" : "http://a3.twimg.com/profile_background_images/138228501/149174881-8cd806890274b828ed56598091c84e71_4c6fd4d8-full.jpg",
        "statuses_count" : 2475,
        "following" : null,
        "profile_text_color" : "362720",
        "protected" : false,
        "show_all_inline_media" : false,
        "profile_background_tile" : true,
        "name" : "Catherine Mullane",
        "contributors_enabled" : false,
        "profile_link_color" : "B40B43",
        "followers_count" : 169,
        "id" : 37486277,
        "profile_use_background_image" : true,
        "utc_offset" : null
    },
    "favorited" : false,
    "in_reply_to_user_id" : null,
    "id" : NumberLong("22819398300")
}

Answer 1

使用 $first 和您的聚合管道查询如下：

db.collectionName.aggregate({
  "$match": {
    "user.statuses_count": {
      "$gt": 99
    },
    "user.time_zone": "Brasilia"
  }
}, {
  "$sort": {
    "user.followers_count": -1 // sort followers_count first 
  }
}, {
  "$group": {
    "_id": "$user.id",
    "followers": {
      "$first": "$user.followers_count" //use mongo $first method to get followers count or max followers count
    },
    "screen_name": {
      "$first": "$user.screen_name"
    },
    "retweet_count": {
      "$first": "$retweet_count"
    }
  }
})

或使用 $limit and $project 作为

db.collectionName.aggregate({
  "$match": {
    "user.statuses_count": {
      "$gt": 99
    },
    "user.time_zone": "Brasilia"
  }
}, {
  "$sort": {
    "user.followers_count": -1 // sort followers_count 
  }
}, {
  "$limit": 1 // Set limit 1 so get max followers_count document first 
}, {
  "$project": { // user project here 
    "userId": "$user.id",
    "screen_name": "$user.screen_name",
    "retweet_count": "$retweet_count"
  }
}).pretty()

Answer 2

以下聚合管道使用 $$ROOT system variable which references the root document, i.e. the top-level document, currently being processed in the $group aggregation pipeline stage. This is added to an array using the $addToSet operator. In the following pipeline stage, you can then $unwind the array to get the desired fields through a $project 运算符修改输出文档的形式：

db.tweet.aggregate([
    {
        '$match': { 
            "user.statuses_count": { "$gte": 100 },
            "user.time_zone": "Brasilia"
        }
    },
    {
        "$group": {
            "_id": "$user.id",
            "max_followers": { "$max": "$user.followers_count" },
            "data": { "$addToSet": "$$ROOT" }
        }
    },
    {
        "$unwind": "$data"
    },   
    {
        "$project": {
            "_id": "$data._id",
            "followers": "$max_followers",
            "screen_name": "$data.user.screen_name",
            "tweets": "$data.user.statuses_count"
        }
    }, 
    {
        "$sort": { "followers": -1 }
    },
    {
        "$limit" : 1
    }
])

以下流水线也取得了相同的结果，但未使用 $group 运算符：

pipeline = [
    {
        "$match": {
            "user.statuses_count": {
                "$gte": 100 
            }, 
            "user.time_zone": "Brasilia"
        }
    },
    {
        "$project": {                
            "followers": "$user.followers_count",
            "screen_name": "$user.screen_name",
            "tweets": "$user.statuses_count"
        }
    },
    {
        "$sort": { 
            "followers": -1 
        }
    },
    {"$limit" : 1}
]

Pymongo 输出:

{u'ok': 1.0,
 u'result': [{u'_id': ObjectId('5304e2d34149692bc5172729'),
              u'followers': 17209,
              u'screen_name': u'AndreHenning',
              u'tweets': 8219}]}

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