Mongoose 8.x: Required array schema validation breaks in static validation

[skrtheboss]

Prerequisites

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

Mongoose version

8.9.3

Node.js version

20.17.0

MongoDB server version

7.0.14

Typescript version (if applicable)

5.5.4

Description

When using a schema with a required array field, static validation using Model.validate() fails when the array is empty, even though instance validation using new Model().validate() passes.

Steps to Reproduce

Run the following code with any 8.x.x version and it will fail the static validation:

const mongoose = require('mongoose');

async function runValidationTest() {
    const barSchema = new mongoose.Schema({
        foo: {
            type: mongoose.Schema.Types.String,
            required: true,
        },
    });

    const fooSchema = new mongoose.Schema({
        bars: {
            type: [barSchema],
            required: true,
        },
    });

    const Model = mongoose.model('FooTest', fooSchema);

    const fooInstance = {
        bars: [],
    };

    console.log('--- Instance Validation (new Model().validate()) ---');
    try {
        await new Model(fooInstance).validate();
        console.log('Instance validation: PASSED ✓');
    } catch (error) {
        console.error('Instance validation: FAILED ✗', error);
    }

    console.log('\n--- Static Validation (Model.validate()) ---');
    try {
        await Model.validate(fooInstance);
        console.log('Static validation: PASSED ✓');
    } catch (error) {
        console.error('Static validation: FAILED ✗', error);
    }
}

// Run the test
runValidationTest().catch(console.error);

Expected Behavior

Both instance validation (new Model().validate()) and static validation (Model.validate()) should pass when the bars array is empty because the array exists and satisfies the required constraint.

[skrtheboss]

Maybe has something in common with this issue: #15075