Filament: Clear Related Field Value on Parent Change

When working with Dependant selects, you can reset the value of child select when the parent is changed. Just look at this scenario:

Forms\Components\Select::make('country_id')
    ->relationship('country', 'name')
    ->required(),
Forms\Components\Select::make('city_id')
    ->label('City')
    ->options(function (Forms\Get $get) {
        return City::where('country_id', $get('country_id'))->pluck('name', 'id');
    })
    ->required(),

This could lead to a user selecting a country, choosing a city, and then changing the country. The city select will still have the old value chosen. This could be a better user experience. To fix this, we can reset the city select when the country select is changed:

Forms\Components\Select::make('country_id')
    ->live()
    ->afterStateUpdated(function (Forms\Set $set) {
        $set('city_id', null);
    })
    ->relationship('country', 'name')
    ->required(),
Forms\Components\Select::make('city_id')
    ->label('City')
    ->live()
    ->options(function (Forms\Get $get) {
        return City::where('country_id', $get('country_id'))->pluck('name', 'id');
    })
    ->required(),

That's it! When the country select is changed, the city will be reset to null, preventing any confusion for the user.