code: func Update(c *gin.Context) {
  	db := c.MustGet("db").(*mgo.Database)
  
  	article := models.Article{}
  	err := c.Bind(&article)
  	if err != nil {
  		c.Error(err)
  		return
  	}
  
  	query := bson.M{"_id": bson.ObjectIdHex(c.Param("_id"))}
  	doc := bson.M{
  		"title":      article.Title,
  		"body":       article.Body,
  		"updated_on": time.Now().UnixNano() / int64(time.Millisecond),
  	}
  	err = db.C(models.CollectionArticle).Update(query, doc)
  	if err != nil {
  		c.Error(err)
  	}
  	c.Redirect(http.StatusMovedPermanently, "/articles")
  }
docs: // Update an article