Why Model.findByIdAndUpdate convert the update from object to array?

[mrdulin]

import mongoose from "mongoose";
import util from 'util';
import { config } from '../../config';

mongoose.set('debug', true);

const Model = mongoose.model('Model', new mongoose.Schema({
  values: [{ key: String }]
}));

(async function main() {
  try {
    await mongoose.connect(config.MONGODB_URI);

    // seed
    const model = await Model.create({ _id: new mongoose.Types.ObjectId(), values: [{ key: 'v1' }, { key: 'v2' }] })

    const doc = await Model.findByIdAndUpdate(model._id, { values: { key: 'v3' } }, { new: true });
    console.log(util.inspect(doc, false, null))

  } catch (error) {
    console.error(error);
  } finally {
    // await Promise.all(['models'].map(c => mongoose.connection.dropCollection(c)))
    await mongoose.connection.close()
  }
})();

Logs:

Mongoose: models.findOneAndUpdate({ _id: ObjectId("64706425de19cdc0b8f49291") }, { '$set': { values: [ { key: 'v3', _id: ObjectId("64706426de19cdc0b8f49296") } ] }}, { returnDocument: 'after', returnOriginal: false })
{
  _id: new ObjectId("64706425de19cdc0b8f49291"),
  values: [ { key: 'v3', _id: new ObjectId("64706426de19cdc0b8f49296") } ],
  __v: 0
}

As you can see, mongoose wrapped the update in $set for me, that's ok. But why it converts the { values: { key: 'v3' } } to { values: [ { key: 'v3', _id: ObjectId("64706426de19cdc0b8f49296") } ] }? It converts the {key: 'v3'} object to an array. I expect the update should be {$set: {values: {key: 'v3'}}}.

I debug step by step but didn't find the reason. Please explain, thanks.

[vkarpov15]

Your schema defines values as an array, so Mongoose casts the value in your update to an array.