我有以下更新路由:

router.put('/:id', upload.single('userImage'), (req, res) => {
    Users.findById(req.params.id)
        .then(user => {
            user.shareEmail = req.body.shareEmail, 
            user.filmmakerQuestion = req.body.filmmakerQuestion,
            user.genres = req.body.genres,
            user.favoriteFilm = req.body.favoriteFilm,
            user.imbd = req.body.portfolio,
            user.portfolio = req.body.portfolio,
            user.aboutYou = req.body.aboutYou,
            user.userImage = req.file.path 
            
        user 
            .save()
            .then(() => res.json("The User is UPDATED succesfully!"))
            .catch(err => res.status(400).json(`Error: ${err}`)); 
        }) 
        .catch(err => res.status(500).json(`Error: ${err}`));  
        console.log(req.body); 
        console.log(req.file); 

}); 

我遇到的问题是，如果我只发送一个更新请求:

{
"shareEmail": "yes,  
"filmmakerQuestion": "no" 
}

它还会覆盖那些未定义的值，并重新设置那些值。因此，如果favoriteFilm，genres，portfolio等之前有值，它们将被覆盖为未定义。所以，虽然我想保持所有其他值之前的样子，但它们现在是未定义的。所以我真正发送的请求看起来更像:

{
 "shareEmail": "yes,  
"filmmakerQuestion": "no", 
"genres": undefined, 
"favoriteFilm": undefined, 
"imbd": undefined, 
"aboutYou": undefined, 
"userImage" : undefined
}

我只想更新指定的字段，其他值保持不变。如何在单个请求中处理此问题？

谢谢！