（MongoDB / Mongoose）在Mongo中将项目从一个属性推到另一个属性

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我有一个用户文档，如下所示：

{
  "id": "12345",
  "favoriteChannels": []
  "channels": [
    {
      "_id": 120398123098120398
      "id": "67890",
      "count": 1
    },
    ...
  ]
}

What I want to do is get the channels array, find the top three channels (by count), and push the {id: channel.id, count: channel.count} objects into User.favoriteChannels.

这是我到目前为止所拥有的：

UserModel.findOne({id:user.id}).then(user => {
  user.favoriteChannels = []
  let channels = user.channels.sort((a, b) => (a.count < b.count) ? 1 : -1).slice(0,3)
  channels.forEach(c => {
    user.favoriteChannels.push({id: c.id, count: c.count})
  })
  user.save(err => { if(err) console.log('Could not save user:', err) })
 })

When I execute this, the document doesn't update, and no error is provided: Could not save user: null.

Interestingly, when I print user.favoriteChannels, I get:

[ { _id: 98349823749822,
    id: '09822',
    messageCount: 0 },
  { _id: 98349823749824,
    id: '12398',
    messageCount: 0 },
  { _id: 98349823749823,
    id: '01983',
    messageCount: 0 } ]

但是...我以为我删除了这些_id？我究竟做错了什么？是否有比我正在尝试的更好或更简单的解决方案？

编辑：修改代码以使内容更加清晰。