const nestedSchema = new mongoose.Schema({
  num: {
    type: mongoose.Types.Decimal128,
    get: (val) => `${val}`
  }
});

const rootSchema = new mongoose.Schema({});
rootSchema.plugin(mongooseLeanGetters);

const discriminatedSchema = new mongoose.Schema({
  discriminatedProp: { type: nestedSchema },
  discriminatedArray: { type: [nestedSchema] },
});

const RootModel = mongoose.model('Root', rootSchema);
const DiscriminatedModel = RootModel.discriminator('Discriminated', discriminatedSchema);

const entry = await DiscriminatedModel.create({
  discriminatedProp: { num: -0.222222222222222222 },
  discriminatedArray: [{ num: -0.333333333333333333 }],
});

const found = await DiscriminatedModel.findById(entry._id).lean({ getters: true }).exec();

assert.equal(typeof found.discriminatedProp.num, 'string');  // fail
assert.equal(typeof found.discriminatedArray[0].num, 'string');  // fail