public function fillKeyField(array &$fields)
    {
        $model   = $this->config->getDataModel();
        $keyName = $model->getKeyName();

        //add the primary key field, which will be uneditable, but part of the data model
        if ($this->config->getType() === 'model' && !isset($fields[$keyName])) {
            $keyField         = $this->make($keyName, array('visible' => false));
            $fields[$keyName] = $keyField->getOptions();
        }
    }