octobercms/october · error · ApplicationException

Class :class must define property $:property used by :behavi

Error message

Class :class must define property $:property used by :behavior behavior.

What it means

ModelBehavior is the base class for model behaviors attached via public $implement (e.g. System\Behaviors\SettingsModel). Its constructor iterates $requiredProperties and checks each against the host model instance with isset(); any missing property throws ApplicationException with the lang key system::lang.behavior.missing_property, naming the model class, the property, and the behavior. SettingsModel for example requires $settingsFields and $settingsCode on the model.

Source

Thrown at modules/system/classes/ModelBehavior.php:32

{
    /**
     * @var array requiredProperties that must exist in the model using this behavior.
     */
    protected $requiredProperties = [];

    /**
     * __construct
     * @param \October\Rain\Database\Model $model The extended model.
     * @throws ApplicationException
     */
    public function __construct($model)
    {
        parent::__construct($model);

        // Validate model properties
        foreach ($this->requiredProperties as $property) {
            if (!isset($model->{$property})) {
                throw new ApplicationException(Lang::get('system::lang.behavior.missing_property', [
                    'class' => get_class($model),
                    'property' => $property,
                    'behavior' => get_called_class()
                ]));
            }
        }
    }
}

View on GitHub (pinned to b608633a7e)

Solutions

1. Add the missing public property to the model class — the message names exactly which property and which behavior demanded it.
2. Ensure the property is public and spelled exactly as required (e.g. $settingsCode, $settingsFields for SettingsModel).
3. For your own behaviors, review requiredProperties and declare each listed key on the consuming model.

Example fix

// before
class Settings extends Model
{
    public $implement = ['System.Behaviors.SettingsModel'];
}

// after
class Settings extends Model
{
    public $implement = ['System.Behaviors.SettingsModel'];
    public $settingsCode = 'acme.blog.settings';
    public $settingsFields = 'settings.yaml';
}

// Before attaching a behavior, confirm the model defines every required property
function behaviorRequirementsMet(string $modelClass, array $requiredProperties): bool
{
    foreach ($requiredProperties as $property) {
        if (!property_exists($modelClass, $property)) {
            return false;
        }
    }
    return true;
}

function hasSettingsBehaviorConfig(string $modelClass): bool
{
    return property_exists($modelClass, 'settingsCode')
        && property_exists($modelClass, 'settingsFields');
}

try {
    $model->addBehavior(System\Behaviors\SettingsModel::class);
} catch (\October\Rain\Exception\ApplicationException $e) {
    // surface to the developer at boot, not at request time
    throw new RuntimeException('Settings model misconfigured: ' . $e->getMessage(), 0, $e);
}

When it happens

Trigger: Declaring public $implement = ['System.Behaviors.SettingsModel']; on a model without also declaring public $settingsCode = 'acme.blog.settings'; and public $settingsFields = 'settings.yaml';; attaching any custom behavior that sets $requiredProperties to a model lacking those public properties.

Common situations: First-time settings-model setup following docs partially; refactoring a settings model and dropping a property; custom behaviors written by plugin authors who list required config keys the consumer forgot; property declared but not public (protected visibility fails isset($model->...) from outside).

Related errors

• Key property ${keyProperty} is not found in the Inspector da
• getDependencies() must return an array: %s
• Vue component class not found: %s
• Vue component class must be a descendant of Backend\Classes\
• A widget class name ':name' has not been registered

AI-assisted analysis of octobercms/october@b608633a7e (2026-08-21). Data as JSON: /api/errors/2595e8bd682bec7d. Report an issue: GitHub.