One to Many Relationship. Implementing --associate

  public function briefcase(): belongsTo
  {
      return $this->belongsTo(Briefcase::class);
  }

  public function businesses(): hasMany
  {
      return $this->hasMany(Business::class);
  }

php artisan make:filament-relation-manager BriefcaseResource businesses name --view --associate

Call to undefined method App\Models\Business::briefcases()