Fix NoMethodError in admin user form validation - handle nil class attribute

[Copilot]

Problem

When creating users through the admin interface (/admin/users) with invalid form data, the application was returning a 500 Internal Server Error instead of displaying helpful validation error messages. This occurred due to a NoMethodError in the form error handling code.

The specific error was:

NoMethodError (undefined method '+' for nil):
config/initializers/action_view_fields_with_errors.rb:8:in 'block in <main>'

This happened when form fields didn't have an existing class attribute, causing element["class"] to return nil, and the code attempted to concatenate a string to nil using the + operator.

Solution

Fixed the field_error_proc in config/initializers/action_view_fields_with_errors.rb to safely handle nil class attributes:

Before (buggy code):

if element && %w[input].include?(element.name)
  element["class"] += " is-invalid"  # ❌ NoMethodError when class is nil
  fragment.to_html.html_safe

After (fixed code):

if element && %w[input].include?(element.name)
  existing_class = element["class"] || ""
  element["class"] = existing_class.empty? ? "is-invalid" : "#{existing_class} is-invalid"
  fragment.to_html.html_safe

Testing

Added comprehensive tests to ensure the fix works correctly:

1. Unit tests for the field error proc covering:

   • Form fields with no class attribute
   • Form fields with empty class attribute
   • Form fields with existing class attributes
   • Non-input elements (should remain unchanged)

2. Integration tests for admin user creation scenarios:

   • Valid user creation (should succeed)
   • Invalid password length (should show validation errors, not 500)
   • Invalid email format (should show validation errors, not 500)
   • Duplicate email addresses (should show validation errors, not 500)

Impact

• ✅ Admin users now see proper validation error messages instead of 500 errors
• ✅ Form fields are properly highlighted with is-invalid CSS class on validation errors
• ✅ Better user experience for admin user management
• ✅ No breaking changes to existing functionality

This fix resolves the core issue where validation failures in the admin interface caused server crashes instead of displaying user-friendly error messages.

Fixes #3700.

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[pglombardo]

Admin interface is getting replaced in v1.63.0 so this PR is unnecessary.