Self populating doesn't work in 6.1.8

[AndreyKomin]

const historySchema = new mongoose.Schema(
  {
    plan: Object,
    parents: [this],
    isDeleted: Boolean,
    isDraft: Boolean,
  },
  {
    timestamps: true,
  }
)

export default mongoose.model("history", historySchema)

Look at parents. It worked perfectly in ^5.13.14 but not in ^6.1.8.
I tried some ways to make it work but unfortunately without any effect.

[vkarpov15]

What is this in your example?

Also, what is the expected behavior versus the observed behavior?

[AndreyKomin]

Think that this is referring to itself.

I want to do await History.findById(historyId).populate("parents") and with the example above it worked perfectly in ^5.13.14. So I was able to populate another history items in parents, referring to itself in the Schema.

Now, in ^6.1.8 it no longer works

[vkarpov15]

"this is referring to itself" <-- no idea what this means. I suspect that this is either null or global object.

Please provide a complete repro script that demonstrates the behavior you're seeing.

[AndreyKomin]

@vkarpov15
Hi there, sorry for delay. Providing the script.
So try to run it under ^5.13.14 and then under ^6.2.2 and then compare output of console.log(result)

const mongoose = require("mongoose")
const MONGO_URI = "mongodb://localhost:27017/test-issue"

const historySchema = new mongoose.Schema({
  data: String,
  parents: [this],
})

const History = mongoose.model("history", historySchema)

const run = async () => {
  await mongoose.connect(MONGO_URI)

  await History.deleteMany({}) // Freshening the collection

  const parentHistoryItem = await History.create({ parents: [], data: "Parent data in here" })
  const parentObjectId = parentHistoryItem._id

  const childHistoryItem = await History.create({ parents: [parentObjectId], data: "Child data" })
  const childObjectId = childHistoryItem._id

  const result = await History.findById(childObjectId).populate("parents")

  console.log(result)
}

;(async function () {
  await run()
})()

[vkarpov15]

@AndreyKomin the script as written worked by accident, and isn't a feature that Mongoose supports. The problem is that, in your script, Mongoose 5 interpretted [this] to be [], an array of any type. The correct way to define your schema is as follows:

const historySchema = new mongoose.Schema({
  data: String,
  parents: [{ type: mongoose.ObjectId, ref: 'history' }],
})

In Mongoose 6, we also don't fall back to using the History model when using Query.prototype.populate(). We'll add a note to our migration guide about this. But, in the meantime, please change how parents is defined in your schema and that should fix this issue.