如何在mongodb中获取数组中元素的数量 我正在使用golang即mgo驱动程序？

I have the following code but I am stuck in getting the the counts of likes as I get nil result.

package main

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

func main() {
    uri := "mongodb://root:tempyml@localhost:27017"

    sess, err := mgo.Dial(uri)
    if err != nil {
        fmt.Printf("Can't connect to mongo, go error %v
", err)
        os.Exit(1)
    }
    defer sess.Close()

    sess.SetSafe(&mgo.Safe{})
    collection := sess.DB("visibl_dev").C("user_videos")

    pipe := collection.Pipe([]bson.M{{"$project": bson.M{"likes": 1,
        "likeCount": bson.M{"$size": "$likes"}}}})
    result := []bson.M{}
    err = pipe.All(&result)
    if err != nil {
        //handle error
    }
    fmt.Println(result)
}