如何使用golang在mongodb中使用$ lookup获取计数值？

Z时代
2024-01-10
分类：问答

使用聚合，我使用golang加入了两个mongodb集合。结果如下所示：-

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": "puneet@bookingkoala.com", "name": "puneet", "post_id": 1, "reply_to": 1524644601, "status": 1 }, { "_id": 7, "comment": "this is a least comment", "comment_on": 1524647808, "email": "puneet@bookingkoala.com", "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": "puneet@bookingkoala.com", "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" } ] } }

此由以下golang代码获取：-

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count的data

了data。在上面显示的输出中意味着在resultField其中显示数据，但是我只需要count像这样的值：-

"resultField":[2]但它正在显示数据。如何获得输出中数据的计数值。先感谢您。

回答：

因此，您的汇总实际上返回comment了该resultField字段中的所有文档，该文档隐含了结果的数量，您可以使用内置len()函数在Go中检查其中的长度。

由于您只需要长度（comment文档数），这就是为什么只想检索此数组的大小的原因。为此，您可以使用$addFields舞台resultField用一个数字替换该阵列的长度，以该阵列的长度。

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"},
        },
    },
})