How to get the sum of a pivot table column in Laravel Filament?

My Workshop model has this relationship to User model. user_workshop being the pivot table.

public function users(): BelongsToMany
  {
    return $this->belongsToMany(User::class, 'user_workshop')
    ->withPivot('num_attending', 'price', 'paid_at');
  }

I would like to get number of places (spots) sold. A user can buy 1 or 2 spots. In Filament table I'm trying to

->sum('users', 'user_workshop.num_attending')

This produces this query

select `workshops`.*, (select sum(`user_workshop`.`num_attending`) from `users` inner join `user_workshop` on `users`.`id` = `user_workshop`.`user_id` where `workshops`.`id` = `user_workshop`.`workshop_id` and `users`.`deleted_at` is null) as `users_sum_users_workshopnum_attending` from `workshops` where `workshops`.`event_id` = 3 and `workshops`.`event_id` is not null limit 10 offset 0

And I get the correct result under users_sum_users_workshopnum_attending when I'm quering the database directly.

In Filament I'm trying this (but the column is blank)

TextColumn::make('users_sum_users_workshopnum_attending')
          ->sum('users', 'user_workshop.num_attending')
          ->label('Sold'),

Any ideas?

To display a count of relationships in the table, you can use ->counts()

TextColumn::make('users_sum_num_attending')->sum('users', 'num_attending'),

for more details, you can check the Filament Column Relationship