Trying to build an array for laravel sync() method using a collection pipeline

controller_snippet.php

/**
  The allocate_ids and remove_ids are from a multiselect so just an array of id's.
  Trying to end up with a structure like :
  [
    123 => ['is_accepted' => true],
    345 => ['is_accepted' => true],
    ...
  ]
  No matter how many variations of map/flatmap/values/blah I used I always either ended up
  with a collection of collections, or had re-keyed the collection to something like :
  [
    0 => ['is_accepted' => true],
    1 => ['is_accepted' => true],
  ]
*/
    public function updateStudents(Request $request, $id)
    {
        $project = Project::findOrFail($id);
        $addIds = collect($request->allocate_ids);
        $currentIds = $project->students->pluck('id');
        $removeIds = collect($request->remove_ids);
        $newIds = $currentIds->merge($addIds)->diff($removeIds);  // ->map/flatMap/argh
        $items = [];
        foreach ($newIds as $item) {
            $items[$item] = ['is_accepted' => true];
        }
        //dd($items);
        $project->students()->sync($items);
        return redirect()->route('project.show', $id);
    }

Thanks go to @adamwathan : https://gist.github.com/adamwathan/a04873b44a1dcd0f2b4257168499162c