DOC Document using symfony/validator (#454)

en/02_Developer_Guides/00_Model/09_Validation.md

@@ -6,6 +6,35 @@ icon: check-square
 
 # Validation and constraints
 
+## Validation using `symfony/validator` constraints {#symfony-validator}
+
+The [`ConstraintValidator`](api:SilverStripe\Core\Validation\ConstraintValidator) class provides an abstraction around [`symfony/validator`](https://symfony.com/doc/current/components/validator.html), so you can easily validate values against symfony's validation constraints and get a [`ValidationResult`](api:SilverStripe\ORM\ValidationResult) object as a result.
+
+```php
+use SilverStripe\Core\Validation\ConstraintValidator;
+
+/**
+ * @var \Symfony\Component\Validator\Constraint $constraint
+ * @var \SilverStripe\ORM\ValidationResult $result
+ */
+$result = ConstraintValidator::validate($valueToValidate, $constraint);
+```
+
+To test if a URL is valid, for example:
+
+```php
+use SilverStripe\Core\Validation\ConstraintValidator;
+use Symfony\Component\Validator\Constraints\Url;
+
+$isValid = ConstraintValidator::validate($url, new Url())->isValid();
+```
+
+You can use most of the constraints listed in Symfony's [supported constraints](https://symfony.com/doc/current/reference/constraints.html) documentation, though note that some of them require additional symfony dependencies.
+
+Validation using constraints that rely on `symfony/doctrine` is explicitly not supported in Silverstripe CMS.
+
+## Model validation
+
 Traditionally, validation in Silverstripe CMS has been mostly handled through [form validation](../forms).
 
 While this is a useful approach, it can lead to data inconsistencies if the record is modified outside of the form context.

en/02_Developer_Guides/03_Forms/01_Validation.md

@@ -6,6 +6,10 @@ icon: check-square
 
 # Form validation
 
+> [!HINT]
+> Before you start implementing custom validation logic, check out [validation using `symfony/validator` constraints](/developer_guides/model/validation/#validation-and-constraints)
+> and see if there's an existing constraint that can do the heavy lifting for you.
+
 Silverstripe CMS provides server-side form validation out of the box through the [Validator](api:SilverStripe\Forms\Validator) abstract class and its' child classes
 (see [available validators](#available-validators) below). A single `Validator` instance is set on each `Form`. Validators are implemented as an argument to
 the [Form](api:SilverStripe\Forms\Form) constructor or through the function `setValidator`.

en/04_Changelogs/5.2.0.md

@@ -13,6 +13,7 @@ title: 5.2.0 (unreleased)
   - [ErrorPage allowed codes configuration](#errorpage-allowed-codes-configuration)
   - [Create random passwords for new users](#create-random-passwords-for-new-users)
   - [Buttons to select all files and deselect all files](#bulk-action-buttons)
+  - [Support for validation using `symfony/validator`](#symfony-validator)
   - [New searchable dropdown fields](#searchable-dropdown-field)
   - [Create file variants with different extensions](#file-variants)
   - [More nuanced permissions for `/dev/*` routes](#dev-route-permissions)
@@ -170,6 +171,33 @@ The files section of the CMS now has buttons to select and deselect all files an
 
 ![select all functionality in the CMS](../_images/asset-admin-select-all.png)
 
+### Support for validation using `symfony/validator`{#symfony-validator}
+
+A new [`ConstraintValidator`](api:SilverStripe\Core\Validation\ConstraintValidator) class has been added which provides an abstraction around [`symfony/validator`](https://symfony.com/doc/current/components/validator.html), so you can easily validate values against symfony's validation constraints and get a [`ValidationResult`](api:SilverStripe\ORM\ValidationResult) object as a result.
+
+```php
+use SilverStripe\Core\Validation\ConstraintValidator;
+
+/**
+ * @var \Symfony\Component\Validator\Constraint $constraint
+ * @var \SilverStripe\ORM\ValidationResult $result
+ */
+$result = ConstraintValidator::validate($valueToValidate, $constraint);
+```
+
+To test if a URL is valid, for example:
+
+```php
+use SilverStripe\Core\Validation\ConstraintValidator;
+use Symfony\Component\Validator\Constraints\Url;
+
+$isValid = ConstraintValidator::validate($url, new Url())->isValid();
+```
+
+You can use most of the constraints listed in Symfony's [supported constraints](https://symfony.com/doc/current/reference/constraints.html) documentation, though note that some of them require additional symfony dependencies.
+
+We explicitly don't support validation using constraints that rely on `symfony/doctrine`.
+
 ### New searchable dropdown fields {#searchable-dropdown-field}
 
 A pair of new dropdown form fields have been added which are particularly useful for dropdowns with a large number of options.