private function loadModels(array $items)
    {
        $classes = [];
        $results = [];

        // Retrieve a list of keys to load from each class.
        foreach ($items as $item) {
            if (!is_null($item['key'])) {
                $classes[$item['class']][] = $item['key'];
            }
        }

        // Iterate list of classes and load all records matching a key.
        foreach ($classes as $class => $keys) {
            $model = new $class();
            $results[$class] = $model->whereIn($model->getKeyName(), $keys)->get()->keyBy($model->getKeyName());
        }

        return $results;
    }