protected function populateAssociatedClassIndex()
    {
        $this->associatedClassIndex = [];

        foreach ($this->modules as $module) {

            if (    ! $module->getAssociatedClass()
                ||  array_key_exists('', $this->associatedClassIndex)
            ) {
                continue;
            }

            $this->associatedClassIndex[ $module->getAssociatedClass() ] = $module->getKey();
        }

        return $this;
    }