The structure is as follows

var LabelSchema = new mongoose.Schema({
  name: String,
  parent: {type: ObjectId, ref: 'Label', default: null},
  children: [{type: ObjectId, ref: 'Label'}]
})

Hope to get the complete tree structure at one time

Label.find({parent: null})
    .populate('children')
    .exec(function(err, labels) {
      if (err) {
        console.log(err)
      }
      // res.send('test')
      res.send({
        msg: true,
        result: labels
      })
    })

The populate method is used, but only the first-level childern reference can be obtained. The second-level childern is still the objectId. In addition to finding the object through the objectId, is there any other easier way to obtain the complete tree structure?