如何在 Go 中實作 io.Writer 介面來記錄到 MongoDB？

import (
    "io"
    "log"

    "gopkg.in/mgo.v2"
)

type MongoWriter struct {
    sess *mgo.Session
}

func (mw *MongoWriter) Write(p []byte) (n int, err error) {
    c := mw.sess.DB("").C("log")
    err = c.Insert(bson.M{
        "created": time.Now(),
        "msg":     string(p),
    })
    if err != nil {
        return
    }
    return len(p), nil
}

func main() {
    // Get a MongoDB session
    sess, err := mgo.Dial("mongodb://localhost:27017")
    if err != nil {
        panic(err)
    }

    // Create a MongoWriter instance
    mw := &MongoWriter{sess}

    // Set the MongoWriter as the output for the default Logger
    log.SetOutput(mw)

    // Log a message that will be inserted into MongoDB
    log.Println("I'm the first log message.")
}