fetchSynced.php

<?php

    /**
     * Takes the results of a relation sync and replaces the ids with model instances.
     *
     * @param array $synced The results of a call to `sync()` on a `BelongsToMany` relation.
     * @param string $foreign Namespaced model name. e.g. from Foo::class
     * @return array $sync, but with ids replaced with model instances
     */
    function fetchSynced(array $synced, string $foreign): array
    {
        /** @var Model $instance */
        $instance = new $foreign;

        $changes = [];
        $keyToType = [];

        foreach ($synced as $type => $keys) {
            $changes[$type] = [];

            foreach ($keys as $key) {
                $keyToType[$key] = $type;
            }
        }

        $fetched = $keyToType ? $foreign::whereIn($instance->getKeyName(), array_keys($keyToType))->get() : new Collection;

        /** @var Model $model */
        foreach ($fetched as $model) {
            $changes[$keyToType[$model->getKey()]][] = $model;
        }

        return $changes;
    }