Cannot override a non-required field from a base schema as of 5.12.7

[sean-daley]

Do you want to request a feature or report a bug?

Bug

What is the current behavior?

If I have a base schema that includes a field with required set to false and I add an additional schema to it which overrides that field to required set to true, this override stops working as of 5.12.7

If the current behavior is a bug, please provide the steps to reproduce.
Here is a script which reproduces the issue

const mongoose = require("mongoose");

const { Schema } = mongoose;

const baseSchema = new Schema({
    username: {
        type: Schema.Types.String,
        required: false
    },
});

const userSchema = new Schema({
    email: {
      type: Schema.Types.String,
      required: true
    },
    username: {
        type: Schema.Types.String,
        required: true
    },
});

const realSchema = baseSchema.clone()
realSchema.add(userSchema)


const User = mongoose.model("User", realSchema);

const main = async () => {
   await mongoose.connect('mongodb://localhost:27017/test', {
    useCreateIndex: true,
    useUnifiedTopology: true,
    useFindAndModify: true,
    useNewUrlParser: true,
    connectTimeoutMS: 3000
});
    const user = new User({
        email: "user@example.com",
    });

    const dbUser = await user.save();
    console.log(dbUser.toObject());
}

main();

What is the expected behavior?
Because we are NOT setting a username here, this should throw an error.
In versions before 5.12.7 this will error out with:

$ node test.js
(node:32903) UnhandledPromiseRejectionWarning: ValidationError: User validation failed: username: Path `username` is required.
    at model.Document.invalidate (/Users/sean/node/mongoose-test/node_modules/mongoose/lib/document.js:2704:32)
    at /Users/sean/node/mongoose-test/node_modules/mongoose/lib/document.js:2524:17
    at /Users/sean/node/mongoose-test/node_modules/mongoose/lib/schematype.js:1256:9
    at process._tickCallback (internal/process/next_tick.js:61:11)

Starting with 5.12.7 though this now succeeds:

$ node test.js
{ _id: 6091342ecfb5858155bdc13e,
  email: 'user@example.com',
  __v: 0 }

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.
Mongoose: 5.12.7
Node: v10.23.0
MongoDB: 4.2.13

[vkarpov15]

Thanks for reporting, fix will be in 5.12.9 👍