PR #12254 introduced bug: Methods defined in shared schemas get removed

Prerequisites

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

Mongoose version

6.5.5

Node.js version

16.6

MongoDB server version

6.0.1

Description

PR https://github.com/Automattic/mongoose/issues/12254 introduces a bug where instance methods defined in shared schema get deleted.

Problem located in line 38 in file mongoose/lib/helpers/model/applyMethods.js

Commit https://github.com/Automattic/mongoose/commit/9fbf92c3a8c6f0717beb404f4020832aafe93589#diff-9de3851c9d06234f6bef122196730d35ce2dc68fc2b609824b1676194608e4fb

Steps to Reproduce

1. Install mongoose 6.5.5
2. Run this bug demo script:

const mongoose = require('mongoose')
mongoose.set('debug', true)

const sharedSubSchema = new mongoose.Schema({
  name: {
    type: String
  }
})

sharedSubSchema.methods.sharedSubSchemaMethod = function () {
  console.log('sharedSubSchemaMethod called')
}

const mainDocumentSchema = new mongoose.Schema({
  subdocuments: {
    type: [sharedSubSchema],
    required: true
  }
})
const MainDocument = mongoose.model('MainDocument', mainDocumentSchema)

const secondaryDocumentSchema = new mongoose.Schema({
  subdocuments: {
    type: [sharedSubSchema],
    required: true
  }
})
const SecondaryDocument = mongoose.model('SecondaryDocument', secondaryDocumentSchema)

const mainDoc = new MainDocument({
  subdocuments: [
    {
      name: 'one'
    }
  ]
})

const secondaryDoc = new SecondaryDocument({
  subdocuments: [
    {
      name: 'secondary'
    }
  ]
})

console.log(mainDoc.subdocuments[0].sharedSubSchemaMethod)
console.log(secondaryDoc.subdocuments[0].sharedSubSchemaMethod)

Expected Behavior

Sub-Document method should be defined on both models, not only one.

Buggy output mongoosejs 6.5.5 [Function (anonymous)] undefined

Correct output mongoosejs 6.5.4 [Function (anonymous)] [Function (anonymous)]

[backflip]

Just stumbled upon the same problem. For reference: The corresponding PR is https://github.com/Automattic/mongoose/pull/12391

[maximilianschmid]

Just to clarify: version 6.6.0 also has this bug.