Unless you like to live dangerously, any Nova fields that are displayed on the Nova creation / update pages will need some validation. Thankfully, it’s a cinch to attach all of the Laravel validation rules you’re familiar with to your Nova resource fields. Let’s get started.

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Rules

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Attaching Rules

When defining a field on a resource, you may use the rules method to attach validation rules to the field:
app/Nova/~Resource.php
use Laravel\Nova\Fields\Text;
use Laravel\Nova\Http\Requests\NovaRequest;

// ...

/**
 * Get the fields displayed by the resource.
 *
 * @return array<int, \Laravel\Nova\Fields\Field>
 */
public function fields(NovaRequest $request): array
{
    return [
        Text::make('Name')
            ->sortable()
            ->rules('required', 'max:255'),
    ];
}
Of course, if you are leveraging Laravel’s support for validation rule objects, you may attach those to resources as well: 
use App\Rules\ValidState;
use Laravel\Nova\Fields\Text;

// ...

Text::make('State')
    ->sortable()
    ->rules('required', new ValidState),
You may also provide rules to the rules method via an array or Closure: 
use App\Rules\ValidState;
use Laravel\Nova\Fields\Text;

// ...

// Using an array...
Text::make('State')->rules(['required', new ValidState]),

// Using a Closure...
Text::make('State')->rules(fn ($request) => [
    'required', 
    new ValidState(),
]);
Additionally, you may use custom closure rules to validate your resource fields: 
use Laravel\Nova\Fields\Text;

// ...

Text::make('State')
    ->sortable()
    ->rules('required', function($attribute, $value, $fail) {
        if (strtoupper($value) !== $value) {
            return $fail('The '.$attribute.' field must be uppercase.');
        }
    }),

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Creation Rules

If you would like to define rules that only apply when a resource is being created, you may use the creationRules method: 
use Laravel\Nova\Fields\Text;

// ...

Text::make('Email')
    ->sortable()
    ->rules('required', 'email', 'max:255')
    ->creationRules('unique:users,email'),

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Update Rules

Likewise, if you would like to define rules that only apply when a resource is being updated, you may use the updateRules method. If necessary, you may use resourceId place-holder within your rule definition. This place-holder will automatically be replaced with the primary key of the resource being updated: 
use Laravel\Nova\Fields\Text;

// ...

Text::make('Email')
    ->sortable()
    ->rules('required', 'email', 'max:255')
    ->creationRules('unique:users,email')
    ->updateRules('unique:users,email,{{resourceId}}'),

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After Validation Hooks

Nova also provides several methods that allow you to perform tasks after a resource has been validated, providing the opportunity to perform more custom validation before the resource is persisted to the database:

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The afterValidation Method

The afterValidation method will always be called after a resource has been validated during its creation or during an update. This method will be called before calling afterCreationValidation or afterUpdateValidation:
app/Nova/~Resource.php
use Illuminate\Contracts\Validation\Validator as ValidatorContract;
use Laravel\Nova\Http\Requests\NovaRequest;

// ... 

/**
 * Handle any post-validation processing.
 *
 * @return void
 */
protected static function afterValidation( 
    NovaRequest $request, 
    ValidatorContract $validator
) {
    if (self::somethingElseIsInvalid()) {
        $validator->errors()->add('field', 'Something is wrong with this field!');
    }
}

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The afterCreationValidation Method

The afterCreationValidation method will be called after a resource that is being created has been validated:
app/Nova/~Resource.php
use Illuminate\Contracts\Validation\Validator as ValidatorContract;
use Laravel\Nova\Http\Requests\NovaRequest;

// ... 

/**
 * Handle any post-creation validation processing.
 * 
 * @return void
 */
protected static function afterCreationValidation(
    NovaRequest $request, 
    ValidatorContract $validator
) {
    if (self::somethingElseIsInvalid()) {
        $validator->errors()->add('field', 'Something is wrong with this field!');
    }
}

​
The afterUpdateValidation Method

The afterUpdateValidation method will be called after a resource that is being updated has been validated:
app/Nova/~Resource.php
use Illuminate\Contracts\Validation\Validator as ValidatorContract;
use Laravel\Nova\Http\Requests\NovaRequest;

// ... 

/**
 * Handle any post-update validation processing.
 *
 * @return void
 */
protected static function afterUpdateValidation(
    NovaRequest $request, 
    ValidatorContract $validator
) {
    if (self::somethingElseIsInvalid()) {
        $validator->errors()->add('field', 'Something is wrong with this field!');
    }
}