如何使用golang在mongodb中使用$lookup获取计数值? [英] How to get the count value using $lookup in mongodb using golang?

问题描述

使用聚合，我正在使用 golang 加入两个 mongodb 集合.结果如下:-

Using aggregation I am joining two collections of mongodb using golang. The result is seems like below:-

输出:-

{
"response": {
    "code": 1,
    "api_status": 1,
    "message": "Success",
    "total_record": [
        {
            "_id": 1,
            "author_name": "mohit",
            "category": 232,
            "content": "This is the content",
            "date_time": 1524632713,
            "excerpt": "This is a  short text",
            "image": "pic.jpg",
            "resultField": [
                {
                    "_id": 6,
                    "comment": "this is a least comment",
                    "comment_on": 1524644601,
                    "email": "[e-mail]",
                    "name": "puneet",
                    "post_id": 1,
                    "reply_to": 1524644601,
                    "status": 1
                },
                {
                    "_id": 7,
                    "comment": "this is a least comment",
                    "comment_on": 1524647808,
                    "email": "[e-mail]",
                    "name": "puneet",
                    "post_id": 1,
                    "reply_to": 1524647808,
                    "status": 1
                }
            ],
            "status": 0,
            "tags": "this",
            "title": "how to do the code"
        },
        {
            "_id": 2,
            "author_name": "mohit",
            "category": 232,
            "content": "This is the content",
            "date_time": 1524632713,
            "excerpt": "This is a  short text",
            "image": "pic.jpg",
            "resultField": [
                {
                    "_id": 8,
                    "comment": "this is a least comment",
                    "comment_on": 1524648059,
                    "email": "[e-mail]",
                    "name": "puneet",
                    "post_id": 2,
                    "reply_to": 1524648059,
                    "status": 1
                }
            ],
            "status": 0,
            "tags": "this",
            "title": "how to do the code"
        },
        {
            "_id": 3,
            "author_name": "puneet",
            "category": 2,
            "content": "this is content",
            "date_time": 1524641086,
            "excerpt": "this is excerpt",
            "image": "pic.jpg",
            "resultField": [],
            "status": 1,
            "tags": "go",
            "title": "how to do the code"
        }
    ]
 }
}

这个输出是由下面的golang代码获取的:-

This output is taken by the below code of golang:-

mongoSession := config.ConnectDb()
collection := mongoSession.DB(config.Database).C(config.BlogCollection)
pipeline := []bson.M{
    bson.M{"$match": bson.M{}},
    bson.M{"$lookup": bson.M{"from" : "comment", "localField" : "_id", "foreignField": "post_id","as": "resultField" }},
}
fmt.Println(pipeline)
pipe := collection.Pipe(pipeline)
resp := []bson.M{}
err = pipe.All(&resp)
if err != nil {
    fmt.Println("Errored: %#v \n", err)
}
fmt.Println(resp)
if err != nil {
    response = ResponseControllerList{
        config.FailureCode,
        config.FailureFlag,
        config.FailureMsg,
        nil,
        nil,
    }
} else {
    response = ResponseControllerList{
        config.SuccessFlag,
        config.SuccessFlag,
        config.SuccessMsg,
        nil,
        resp,
    }
}

问题是:- 我只需要 data 的 count 不需要 data.表示在如上所示的输出中有 resultField 显示数据，但我只需要 count 值，如:- "resultField":[2] 但它正在显示数据.如何获得输出中数据的计数值.提前致谢.

Issue is:- I need only count of the data not the data. Means in the output as shown above there is the resultField in which data is show but I only need the count value like:- "resultField":[2] but it is showing the data. How can I will get the count values of the data in output. Thank you in advance.

推荐答案

所以你的聚合实际上返回了 resultField 字段中的所有 comment 文档，其中隐式包含了结果，它是一个切片，您可以使用内置的 len() 函数在 Go 中检查其中的长度.

So your aggregation actually returns all comment documents in the resultField field, which implicitly contains the number of results, it's a slice of which you can check the length in Go using the builtin len() function.

由于您只需要长度(comment 文档的数量)，这就是为什么您只想检索此数组的大小.为此，您可以使用 $addFields 阶段将 resultField 数组替换为该数组的长度.

Since you only need the length (number of comment documents), that's why you want to only retrieve the size of this array. For that purpose you may use the $addFields stage to replace the resultField array with a number being the length of this array.

pipe := c.Pipe([]bson.M{
    {
        "$lookup": bson.M{
            "from":         "comment",
            "localField":   "_id",
            "foreignField": "post_id",
            "as":           "resultField",
        },
    },
    {
        "$addFields": bson.M{
            "resultField": bson.M{"$size": "$resultField"},
        },
    },
})

请注意，$addFields 阶段等效于 $project 阶段，它明确指定输入文档中的所有现有字段并添加新字段.仅自 MongoDB 3.4 版起可用.

Note that $addFields stage is equivalent to a $project stage that explicitly specifies all existing fields in the input documents and adds the new fields. Available only since MongoDB version 3.4.

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