mongodb聚合在golang中给出错误

Question

I want group methods types and count them according to their types.these methods are strings. I wrote below code.But it gives an error.

pipeline := []bson.D{
        bson.D{
            {"$unwind", "$method"},
        },
        bson.D{
            {"$group", bson.M{"_id": "$method", "count": bson.M{"$sum": 1}}},
        },
query := bson.D{
        {"aggregate", "API_ACCESS_LOGS"}, // useragents is a collection name
        {"pipeline", pipeline},
    }
  err = session.DB("vamps-logs").Run(query, &methods)

It gives below error.

2018/02/06 09:58:33 http: panic serving 127.0.0.1:53973: runtime error: index out of range goroutine 92 [running]:

please help me to correct this

Answer 1

Hi this is a sample code I modified to use pipelines with MgO golang. Watch It and try to apply it. package main

import ("fmt"
    "log"
    "gopkg.in/mgo.v2"
    "gopkg.in/mgo.v2/bson"
)

type Method struct {
        Type string `json:"Type" bson:"Type"`
        Host float64 `json:"Host" bson:"Host"`
}

type Out struct {
        Type string `json:"Type" bson:"Type"`
        Count float64 `json:"count" bson:"count"`
}

func main() {

        session, err := mgo.Dial("localhost")
        if err != nil {
                panic(err)
        }
        defer session.Close()

        // Optional. Switch the session to a monotonic behavior.
        session.SetMode(mgo.Monotonic, true)

        c := session.DB("test").C("api")
        // err = c.Insert(&Method{"GET", 1.0},
           //         &Method{"PUT", 2.0},
           //         &Method{"POST", 2.0})
        if err != nil {
                log.Fatal(err)
        }

        pipe := c.Pipe(
                []bson.M{
                    bson.M{
                        "$unwind":"$Type",
                    },
                    bson.M{
                        "$group":bson.M{
                        "_id":"$Type", 
                        "count":bson.M{"$sum":1,},
                        },
                    },
                    bson.M{
                        "$project":bson.M{
                        "Type":"$_id" , "count":"$count",
                        },
                    },
                },
            )

        result := []Out{}
        // err = c.Find(bson.M{"Type": "GET" }).One(&result)
            err = pipe.All(&result)

        if err != nil {
                log.Fatal(err)
        }

        fmt.Println("Type:", result)
}

Dataset in the mongoDB

/* 1 */
{
    "_id" : ObjectId("5a794efd49d755038cc60307"),
    "Type" : "GET",
    "Host" : 1.0
}

/* 2 */
{
    "_id" : ObjectId("5a794efd49d755038cc60308"),
    "Type" : "PUT",
    "Host" : 2.0
}

/* 3 */
{
    "_id" : ObjectId("5a794efd49d755038cc60309"),
    "Type" : "POST",
    "Host" : 2.0
}

/* 4 */
{
    "_id" : ObjectId("5a794f3149d755038cc6031a"),
    "Type" : "GET",
    "Host" : 1.0
}

/* 5 */
{
    "_id" : ObjectId("5a794f3149d755038cc6031b"),
    "Type" : "PUT",
    "Host" : 2.0
}

/* 6 */
{
    "_id" : ObjectId("5a794f3149d755038cc6031c"),
    "Type" : "POST",
    "Host" : 2.0
}

Out put

Type: [{POST 2} {PUT 2} {GET 2}]