Issue accessing subdocument contents from custom setter

[asabhaney]

Thanks for all the great work on mongoose.

I'd like to report (what I believe to be) a bug, though it's certainly possible I'm trying to do something that is unsupported or that I've misunderstood something. I'm doing some work with single-nested subdocuments, and running in to an issue when I attempt to access the contents of a subdocument from a setter that I've added to the subdoc's schema.

The reason I want to have the custom setter is to be able to access the current value of that property, and set the new value of that property accordingly.

I've written a simple script that reproduces the issue:

const mongoose = require('mongoose')
const Schema = mongoose.Schema

// The subdocument schema
const contentSchema = new Schema({
   blocks: [{ type: String }],
   summary: { type: String }
})

// Subdocument setter
contentSchema.path('blocks').set(function (srcBlocks) {
   const content = this

   // The issue is most obvious when the document is being updated, 
   // so we only focus on the case where the doc is being updated
   if (!content.ownerDocument().isNew) {

      // Ideally I would return something here depending on the current 
      // value of `content.blocks`

      // The issue is here! It seems that accessing `this` shows
      // the subdocument to be empty
      console.log('Normal way', content)

      // ...However, if we try to access the subdocument this way,
      // the subdocument looks just fine!
      console.log('Hack way', content.ownerDocument().get('body'))
   }

   return srcBlocks
})

// Normal schema that uses the above subdocument schema as single nested subdocs
const noteSchema = new Schema({
   title: { type: String, required: true },
   body: { type: contentSchema }
})

const Note = mongoose.model('note-subdoc-test', noteSchema)

// Create a new Note, this works fine
const note = new Note({
   title: 'Lorem Ipsum Dolor',
   body: {
      summary: 'Summary Test',
      blocks: ['html']
   }
})

note.save()
.then(note => Note.findById(note.id))
.then((note) => {
   // This triggers the custom setter, which is where the issue is
   note.set('body', {
      summary: 'New Summary',
      blocks: ['gallery', 'html']
   })

   return note.save()
})

The output of this snippet is:

Normal way { _id: 5941a9d827dd6fb24f82881c, blocks: [] }
Hack way { summary: 'Summary Test',
  _id: 5941a9d627dd6fb24f82881a,
  blocks: [ 'html' ] }

(Even the _id is different - odd!)

Using version 4.10.6 of mongoose and version 6.11.0 of node.js. Thanks in advance!

[vkarpov15]

Thanks for the detailed repro script, will investigate for next patch release.