Follow up to "populated documents get saved as strings"

[fardolieri]

Prerequisites

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

Mongoose version

8.5.2

Node.js version

20.15.1

MongoDB server version

7.0.12

Typescript version (if applicable)

No response

Description

There is an edge case that we missed in #14759. When calling populate on a document the _id is again stored as a string in the DB.

I was able to track it down to this line. But this may not be the whole picture.

mongoose/lib/helpers/populate/getModelsMapForPopulate.js

Line 629 in 9572cbd

return val._id;

 function convertTo_id(val, schema) { 
   if (val != null && val.$__ != null) { 
-    return val._id; 
+    return val._doc._id;
   } 

Steps to Reproduce

It's the same example as in #14759 but I added pet.populate('owner') before pet.save().

mongoose.Schema.ObjectId.get(
  value => {
    if (value?.constructor?.name?.toLowerCase() === 'objectid') return String(value)
    return value
  }
)

const ownerSchema = new mongoose.Schema({ name: 'String' })
const petSchema = new mongoose.Schema({
  name: 'String',
  owner: { type: 'ObjectId', ref: 'owner' }
})

const Owner = mongoose.model('owner', ownerSchema)
const Pet = mongoose.model('pet', petSchema)

const owner = new Owner({ name: 'Alice' })
const pet = new Pet({ name: 'Kitty', owner: owner })

await owner.save()

console.log(typeof pet._doc.owner.$__.wasPopulated.value) // object 😀
await pet.populate('owner') // This breaks it 💥💥💥💥💥💥💥💥💥💥
console.log(typeof pet._doc.owner.$__.wasPopulated.value) // string 🙁

await pet.save()

/**
 * The property 'owner' of 'pet' has unexpectedly been saved
 * as a string into the mongoDB instead of ObjectId.
 * 
 * No matter how I query for it, mongoose is not
 * able to find it...
 */

const reloadAttempts = await Promise.all([
  Pet.findOne({ owner: owner }),
  Pet.findOne({ owner: owner._id }),
  Pet.findOne({ owner: String(owner._id) }),
  Pet.findOne({ owner: new mongoose.Types.ObjectId(owner._id) }),
])

console.log(reloadAttempts) // Logs [ null, null, null, null ] but shouldn't

Expected Behavior

No response