Getting unique models in nested Many To Many Relationship

Hi fellow StackOverflow users!

I'm struggling on a pretty simple task, and I'd be very greetful if one of you could help me out. I have 3 models with nested Many To Many Relationship.

My first model is Wave that has a M2M relationship with Employee :

Wave.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Wave extends Model
{

    public function employees()
    {
        return $this->belongsToMany('App\Employee');
    }

Employee that has a Many To Many with Wave and another Many To Many with Expertise

Employee.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Employee extends Model
{

    public function waves()
    {
        return $this->belongsToMany('App\Wave');
    }

    public function expertises()
    {
        return $this->belongsToMany('App\Expertise');
    }

Finally my Expertise model with a many to many with Employee:

Expertise.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Expertise extends Model
{

    public function employees()
    {
        return $this->belongsToMany('App\Employee');
    }

What I'd like to achieve here is create a custom property on Wave to get all distinct expertises that employees assigned to this wave possess.

It would look a one dimensional collection of distinct expertises.

It doesn't seem very complicated but I tried many things and can't get it working.

Last thing I tried was querying Expertise model using with and withPivotIn but without success:

Wave.php

    public function getExpertisesAttribute()
    {
        $employees = $this->employees;
        $expertises = Expertise::with('employees', function($q) use($employees) {
            $q->wherePivotIn('employee_id', $employees->pluck('id')->toArray());
        });
        return $expertises;
    }

I'm getting an error while running this query:

PHP Warning: mb_strpos() expects parameter 1 to be string, object given in /Users/Kaz/lab/MyProject/vendor/laravel/framework/src/Illuminate/Support/Str.php on line 107

FYI i'm using Laravel 5.8 on PHP 7.2

Thank you very much for reading this and I hope you'll be able to give me some help because I'm feeling a bit stuck and still it seems like an easy thing to do !

via Chebli Mohamed