如何使用golang在mongodb中使用$ lookup获取计数值？

Question

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

OUTPUT:-

{
"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"
        }
    ]
 }
}

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 
", 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,
    }
}

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.

Answer 1

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.

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"},
        },
    },
})

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.