How to Update a Derived Field in a Filament Form

Last updated on August 21st, 2026 at 01:06 pm

The main lesson: when one Filament form field depends on another, make the source field live and update the derived field explicitly. Keep the derived value read-only and exclude it from saved form data.

What I learned

I needed a select field to show a related slug after the user chose a record. The value also needed to be easy to copy, but it should not be written back to the database.

The reliable pattern has four parts:

1. Make the select reactive with live().
2. Use afterStateUpdated() to respond to the new selection.
3. Look up the related value with every key needed to identify the record.
4. Place the result into a disabled, non-dehydrated text input.

Example

Select::make('destination_id')
    ->live()
    ->afterStateUpdated(function ($state, $record, Set $set): void {
        $slug = filled($state) && $record
            ? Item::query()
                ->where('group_id', $record->group_id)
                ->where('item_id', $state)
                ->value('slug')
            : null;

        $set('destination_slug', $slug);
    });

TextInput::make('destination_slug')
    ->disabled()
    ->dehydrated(false)
    ->copyable()
    ->columnSpanFull();

dehydrated(false) means Filament can display the value without including it in the data saved by the form.

Why the full lookup scope matters

A selected ID is not always globally unique. It may only be unique inside a parent record, tenant, site, or group.

Cause: querying with only the selected ID can return the wrong row.

Fix: include the parent key and the selected key in the query. If the database permits duplicate IDs across groups, both conditions are essential.

Common mistakes

• The value does not update: add live() to the source field.
• The derived field disappears: render it unconditionally and set its value to null when no option is selected.
• The wrong record is loaded: include the complete lookup scope.
• The form tries to save the display value: use dehydrated(false).
• The field is hard to use: make it copyable when users need the value elsewhere.

Practical checklist

1. Choose which field triggers the update.
2. Mark that field as live.
3. Query the related value with all required keys.
4. Set the display field from the update callback.
5. Test selection, clearing, copying, and saving.

Conclusion

A derived Filament field is simplest when it is treated as display state, not persisted data. The next action is to test both a valid selection and a cleared selection before shipping the form.

Tags:

• admin
• Development
• laravel
• php