Subdocument Hooks not triggered in Discriminators

[kticka]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Mongoose version

8.8.4

Node.js version

22.12.0

MongoDB server version

8.0.3

Typescript version (if applicable)

No response

Description

Hi, it seems, that hooks of embedded documents aren't triggered properly when used with discriminators. I wrote very simple example to reproduce.

Steps to Reproduce

Document.js

const mongoose    = require('mongoose')
const Subdocument = require('./Subdocument')

const schema = mongoose.Schema({
  name:         String,
  subdocuments: [Subdocument]
}, {discriminatorKey: 'type'})

schema.pre('save', function (next) {
  console.log(`Saving Document: ${this.name}`)
  next()
})

module.exports = mongoose.model('Document', schema)

Subdocument.js:

const mongoose = require('mongoose')

const schema = mongoose.Schema({
  name: String
})

schema.pre('save', function(next) {
  console.log(`Saving Subdocument: ${this.name}`)
  next()
})

module.exports = schema

Discriminator.js:

const mongoose = require('mongoose')
const Document = require('./Document')

const schema = mongoose.Schema({})

schema.pre('save', function(next) {
  console.log(`Saving Discriminator: ${this.name}`)
  next()
})

module.exports = Document.discriminator('Discriminator', schema)

Test.js

module.exports      = async function () {
  const document     = new Document({name: 'Document'})
  const dicriminator = new Discriminator({name: 'Discriminator', type: 'Discriminator'})

  document.subdocuments.push({name: 'Document Subdocument'})
  dicriminator.subdocuments.push({name: 'Discriminator Subdocument'})

  await document.save()
  await dicriminator.save()

  return 'Done'
}

Expected Behavior

When saving either Document or Discriminator, the Subdocument hook should be triggered. The expected output is:

Saving Subdocument: Document Subdocument
Saving Document: Document
Saving Subdocument: Discriminator Subdocument
Saving Document: Discriminator
Saving Discriminator: Discriminator

However, when saving the Discriminator, the Subdocument hook is not triggered. The actual output is:

Saving Subdocument: Document Subdocument
Saving Document: Document
Saving Document: Discriminator
Saving Discriminator: Discriminator

[kticka]

After further testing, I found that the issue does not occur in Mongoose 8.8.2. The problem starts in version 8.8.3 and persists in 8.8.4.

[maximilianschmid]

The problem starts with version 8.8.3 and is still there in 8.8.4 and 8.9. In my case the supdocuments with discriminators don't get the modified date updated.

[vkarpov15]

Confirmed that this issue will be fixed by #15099.