Validation Bypass when Union of Objects is used in Schema

[timheerwagen]

Prerequisites

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

Mongoose version

8.19.3

Node.js version

24.11.0

MongoDB server version

"mongodb-memory-server": "^10.3.0"

Typescript version (if applicable)

No response

Description

If a union of objects is used inside a schema and an arbitrary object is used upon creation, the first Object Schema is used but validation is skipped, at least for the required fields.

Defined field types still validate correctly, and it is not possible to save arbitrary fields.

The same thing happens when you use this with arrays of objects.

Steps to Reproduce

import mongoose from "mongoose";
import { MongoMemoryServer } from "mongodb-memory-server";
import { inspect } from "node:util";

const SubSchema1 = new mongoose.Schema({
  price: { type: Number, required: true },
  title: { type: String },
  isThisSchema1: { type: Boolean },
});

const SubSchema2 = new mongoose.Schema({
  description: { type: String, required: true },
  title: { type: String },
  isThisSchema2: { type: Boolean },
});

const TestSchema = new mongoose.Schema({
  product: {
    type: mongoose.Schema.Types.Union,
    of: [SubSchema1, SubSchema2],
  },
});

const TestModel = mongoose.model("test", TestSchema);

const main = async () => {
  const mongod = await MongoMemoryServer.create();
  const uri = mongod.getUri();
  await mongoose.connect(uri);

  // This should produce an error, since price or description is required.
  await TestModel.create({
    product: {
      title: "string",
      arbitraryNeverSave: true,
      isThisSchema1: true,
      isThisSchema2: true,
    },
  });

  const data = await TestModel.find().lean();

  console.log(inspect(data, { depth: Infinity }));

  process.exit();
};

main();

Expected Behavior

The example should produce an error because the price or description should be defined.

[codervipul775]

Hey @vkarpov15 , I would like to work on this issue. Please assign it to me.

[codervipul775]

Hey @vkarpov15 , I have fixed this issue and raised the PR #15734 . Please review it and merge it if implemented correctly.