猫鼬不是填充嵌套对象吗？

Question

在此解析器中，我无法填充嵌套在帖子“收藏夹”数组中的对象“作者”：

updateFavourites: async ({ _id, post, action }, req) => {
    if (!req.isAuth) {
      throw new Error("Not Authenticated!")
    }
    try {
      const user = await User.findOne({ _id: _id }).populate([
        {
          path: 'favourites',
          model: 'Post',
          populate: {
            path: 'author',
            model: 'User',
          }
        },
      ])
      if (!user) throw new Error("A User by that ID was not found!")

      const postTest = await Post.findOne({ _id: post })
      if (!postTest) throw new Error("A Post by that ID was not found!")

      if (action === "add") {
        user.favourites.forEach(fav => {
          if (post.toString() === fav._id.toString()) {
            throw new Error("Duplicate Favourite!")
          }
        })
        user.favourites.push(post)
      } else {
        user.favourites.pull(post)
      }

      user.updated_at = moment().format()
      await user.save()

      return {
        ...user._doc
      }
    } catch (err) {
      throw err
    }
  },

如果我在user.save（）之后第二次检索到用户，则可以得到想要的结果，但我宁愿通过一次调用数据库来实现：

updateFavourites: async ({ _id, post, action }, req) => {
  if (!req.isAuth) {
    throw new Error("Not Authenticated!")
  }
  try {
    const user = await User.findOne({ _id: _id })
    if (!user) throw new Error("A User by that ID was not found!")

    const postTest = await Post.findOne({ _id: post })
    if (!postTest) throw new Error("A Post by that ID was not found!")

    if (action === "add") {
      user.favourites.forEach(fav => {
        if (post.toString() === fav._id.toString()) {
          throw new Error("Duplicate Favourite!")
        }
      })
      user.favourites.push(post)
    } else {
      user.favourites.pull(post)
    }

    user.updated_at = moment().format()
    await user.save()

    const newUser = await User.findOne({ _id: _id }).populate([
      {
        path: 'favourites',
        model: 'Post',
        populate: {
          path: 'author',
          model: 'User',
        }
      },
    ])
    return {
      ...newUser._doc
    }
  } catch (err) {
    throw err
  }
},

我要传递_id（即用户_id），post（即发布id_id）和action（其是字符串），并根据字符串值从收藏夹数组中推送或拉出帖子。