Calling sortBy with a flat array allows sorting asc by multiple attributes.

The method works but not documented, causing confusion by explicit directions or workarounds like chaining sortBy in reverse order, i.e. $collection->sortBy('second-priority')->sortBy('first-priority'), which does not happen in similir Builder::orderBy method.

Example

> collect([['order_index'=>3,'id'=>4],['order_index'=>2,'id'=>1],['order_index'=>1,'id'=>3],['order_index'=>1,'id'=>2]])->sortBy(['order_index','id'])
= Illuminate\Support\Collection {#7016
    all: [
      3 => [
        "order_index" => 1,
        "id" => 2,
      ],
      2 => [
        "order_index" => 1,
        "id" => 3,
      ],
      1 => [
        "order_index" => 2,
        "id" => 1,
      ],
      0 => [
        "order_index" => 3,
        "id" => 4,
      ],
    ],
  }

Why it works?

In Collection::sortByMany, When passing each comparisons,

• Arr::wrap is called, which wraps string to array
• 1st index of array would be undefined, but it defaults to true for $ascending

https://github.com/laravel/framework/blob/37455bbd9ece2ab48443b4ad2af85abf2140e326/src/Illuminate/Collections/Collection.php#L1458-L1460