How to sort a grouped collection?

Question

Laravel

Laravel 5.5
There is a collection of different types of models:

модель Участник [
type = {школьник или студент}
year = год
olimp_id = связь с моделью олимпиады
rating = результат в баллах
position = место в рейтинге
]

It is necessary to put down each of his place in the ranking for the year-Olympiad-category-rating.
It seems that groupBy() is ideal, but at the last stage it groups the models according to their rating into discord: sorting through sortBy() does not help, because of this, naturally, the position is incorrect. Can you tell me what I can come up with or maybe I went the wrong way?

$rating = $this->rating->groupBy(['year','olimp_id', 'model_type', 'rating']);

$rating->each(function ($year_item, $year) {
  $year_item->each(function ($olimp_item) {
    $olimp_item->each(function ($model_type_item) use(&$position) {
    $position = 0;
      $model_type_item->each(function ($rating_item) use(&$position) {
        $position++;
        $rating_item->each(function ($model) use($position) {
          $model->update(['position' => $position]);
        });
      });
    });
  });
});

Answer 1

$model_type_item->sortByDesc(function ($item) {
  return $item->first()->rating;
})->each(function ($rating_item) use(&$position) {

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