A many-to-many polymorphic relations example of Laravel.

Media.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Media extends Model {}

Post.php

<?php

namespace App;

use App\PostHasMedia;
use Illuminate\Database\Eloquent\Model;

class Post extends Model
{
    public function media()
    {
        return $this->morphToMany(
            Media::class,
            'model',
            PostHasMedia::class,
            'post_id',
            'media_id'
        );
    }
}

PostHasMedia.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Relations\MorphPivot;

// Ref: https://github.com/laravel/framework/issues/21065#issuecomment-327836778
class PostHasMedia extends MorphPivot {}