Fix: Required validation for Ninja Forms integration

[faisalahammad]

🎯 Summary

Fixes an issue where the "Mailchimp opt-in" field in Ninja Forms would not prevent form submission when marked as "Required" but left unchecked. This PR implements strict validation logic for the checkbox field.

📋 Issue Reference

Fixes #717

🔍 Problem Description

Current Behavior

In Ninja Forms, when the "Mailchimp for WordPress" checkbox field is added to a form and the "Required" toggle is enabled, users can still submit the form without checking the box. This happens because Ninja Forms treats an unchecked checkbox (value 0) as a valid submission unless specific validation logic overrides it.

Expected Behavior

If the field is marked as "Required", the form submission should be blocked with an error message ("This field is required") if the checkbox is not checked.

Root Cause

The MC4WP_Ninja_Forms_Field class extends NF_Abstracts_Input but did not implement a custom validate() method to handle the specific behavior of unchecked checkboxes (which submit a value of 0, passing the default empty check in some contexts or failing to trigger the required error correctly).

✨ Solution Overview

Approach Taken

I implemented a validate() method in MC4WP_Ninja_Forms_Field that strictly checks:

1. If the field is marked as required.
2. If the submitted value is empty or 0.

If both are true, it returns a validation error.

Why This Approach

This leverages Ninja Forms' native validation pipeline. By overriding the validate method in the field class itself, we ensure the rule is enforced consistently whenever the field is used, without relying on fragile frontend JavaScript or external hooks.

🔧 Changes Made

Files Modified

• integrations/ninja-forms/class-field.php - Added validate() method to enforce required state.
• tests/NinjaFormsValidationTest.php - (New) Added unit tests to verify usage.

Detailed Changes

1. integrations/ninja-forms/class-field.php

Before:
No validate() method existed; it relied on the parent class which didn't handle the 0 value correctly for this specific required context.

After:

public function validate($field, $data)
{
    $errors = parent::validate($field, $data);

    if (isset($field['required']) && 1 == intval($field['required']) && empty($field['value'])) {
        $errors['slug']    = 'required-error';
        $errors['message'] = esc_html__('This field is required.', 'mailchimp-for-wp');
    }

    return $errors;
}

Why This Works:
It explicitly checks the required flag and the value. Since unchecked checkboxes submit 0 or empty string, empty($field['value']) correctly identifies the invalid state.

🧪 Testing Performed

Test Environment

• WordPress Version: 6.6.1
• PHP Version: 8.1
• Ninja Forms: Latest
• Mailchimp for WordPress: Local build from feature branch

Manual Testing

Test Case 1: Required Field Validation

Steps:

1. Add "Mailchimp opt-in" field to a Ninja Form.
2. Enable "Required" setting.
3. Submit form without checking the box.

Expected Result: Validation error "This field is required" appears.
Actual Result: ✅ Validation error appeared and submission was blocked.

Test Case 2: Successful Submission

Steps:

1. Check the "Mailchimp opt-in" box.
2. Submit form.

Expected Result: Form submits successfully.
Actual Result: ✅ Form submitted.

Automated Testing

$ ./vendor/bin/phpunit tests/NinjaFormsValidationTest.php
PHPUnit 9.6.34 by Sebastian Bergmann and contributors.

...                                                                 3 / 3 (100%)

Time: 00:00.004, Memory: 6.00 MB

OK (3 tests, 5 assertions)

Test Coverage:

• ✅ Validates required field fails when empty/0.
• ✅ Validates required field passes when checked (1).
• ✅ Validates optional field passes when unchecked (0).

⚠️ Breaking Changes

✅ No breaking changes

Screen recording

Plugin build/zip

Uploading mailchimp-for-wp-issue-717.zip…

📚 Documentation Updates

• N/A

✅ PR Checklist

• Code follows WordPress Coding Standards
• All functions have proper PHPDoc blocks
• Input sanitized, output escaped
• Strings are translatable
• Tests pass successfully
• Self-reviewed for quality

[dannyvankooten]

Merged. Thank you @faisalahammad!