…cked for uniqueness (wkania)This PR was squashed before being merged into the 6.1 branch.Discussion----------[Validator] Define which collection keys should be checked for uniqueness| Q             | A| ------------- | ---| Branch?       | 5.4| Bug fix?      | no| New feature?  | yes| Deprecations? | no| Tickets       |Fix#9888| License       | MIT| Doc PR        |symfony/symfony-docs#16713Currently, the validator checks each element of the collection as a whole. We already have a custom normalizer (which is great), but it would be nice to be able to check for uniqueness certain [collection](https://symfony.com/doc/current/reference/constraints/Collection.html) keys.For example, some fields in the collection element can be identifiers. They should be unique within the collection, even when the rest of the element data are different.Current state:- validates that all the elements of the given collection are uniqueNew state:- preserve the current state,- all old tests pass (no changes in them),- no breaking changes,- define which collection fields should be checked for uniqueness (optional),- fields are optional in each element of the collection. Use [collection constraints](https://symfony.com/doc/current/reference/constraints/Collection.html) if they are requiredExamples:1. Basic example. Each translation of the same resource must be in a different language.```phpuse Symfony\Component\Validator\Constraints as Assert;/** *@Assert\Count(min=1), *@Assert\Unique(fields={"language"}), *@Assert\Collection( *         fields = { *             "language" = { *@Assert\NotBlank, *@Assert\Length(min = 2, max = 2), *@Assert\Language *             }, *             "title" = { *@Assert\NotBlank, *@Assert\Length(max = 255) *             }, *             "description" = { *@Assert\NotBlank, *@Assert\Length(max = 255) *             } *         } * ) */public array $translations = [];```2. An example where Optional is recognizable. Items with the id are changed and without are new.```phpuse Symfony\Component\Validator\Constraints as Assert;use Symfony\Component\Validator\Constraints\Optional;/** *@Assert\Unique(fields={"id"}), *@Assert\Collection( *         fields = { *             "id" =@Assert\Optional({ *@Assert\Uuid *             }), *             "name" = { *@Assert\NotBlank, *@Assert\Length(max = 255) *             } *         } * ) */public array $items = [];```3. An example with composite uniqueness```phpuse Symfony\Component\Validator\Constraints as Assert;/** *@Assert\Unique(fields={"latitude", "longitude"}), *@Assert\Collection( *         fields = { *             "latitude" = { *@Assert\NotBlank *             }, *             "longitude" = { *@Assert\NotBlank *             }, *             "poi" = { *@Assert\Length(max = 255) *             } *         } * ) */public array $coordinates = [];```Commits-------0e8f4ce [Validator] Define which collection keys should be checked for uniqueness

…ys (Brajk19)This PR was squashed before being merged into the 6.4 branch.Discussion----------[Validator] UniqueValidator - normalize before reducing keys| Q             | A| ------------- | ---| Branch?       | 6.4| Bug fix?      | yes| New feature?  | no| Deprecations? | no| Issues        || License       | MITIn#42403 checking for uniqueness of certain collection keys was enabled. Method `UniqueValidator::reduceElementKeys` removes all keys which are not specified.Problem is that this happens before normalization, which in my opinion is not great because that method accepts array argument and if i have some object (DTO), TypeError will be thrown.Example:```phpclass ParentDTO{    /**     * `@var` ChildDTO[]     */    #[Assert\Unique(        normalizer: [ChildDTO::class, 'normalize']        fields: 'id'    )]    public array $children;}``````phpclass ChildDTO{    public string $id;    public string $name;    public static function normalize(self $obj): array    {        return [            'id' => $obj->id,            'name' => $obj->name        ];    }}```Because normalization will happen after `reduceElementKeys` this will be thrown:`TypeError: Symfony\Component\Validator\Constraints\UniqueValidator::reduceElementKeys(): Argument#2 ($element) must be of type array, ...\ChildDTO given, called in .../UniqueValidator.php on line 48`If `$element = $normalizer($element);` is executed before `reduceElementKeys` it would enable using Assert\Unique with array of objects when correctly normalizedCommits-------77df90b [Validator] UniqueValidator - normalize before reducing keys

…ys (Brajk19)This PR was squashed before being merged into the 6.4 branch.Discussion----------[Validator] UniqueValidator - normalize before reducing keys| Q             | A| ------------- | ---| Branch?       | 6.4| Bug fix?      | yes| New feature?  | no| Deprecations? | no| Issues        || License       | MITInsymfony/symfony#42403 checking for uniqueness of certain collection keys was enabled. Method `UniqueValidator::reduceElementKeys` removes all keys which are not specified.Problem is that this happens before normalization, which in my opinion is not great because that method accepts array argument and if i have some object (DTO), TypeError will be thrown.Example:```phpclass ParentDTO{    /**     * `@var` ChildDTO[]     */    #[Assert\Unique(        normalizer: [ChildDTO::class, 'normalize']        fields: 'id'    )]    public array $children;}``````phpclass ChildDTO{    public string $id;    public string $name;    public static function normalize(self $obj): array    {        return [            'id' => $obj->id,            'name' => $obj->name        ];    }}```Because normalization will happen after `reduceElementKeys` this will be thrown:`TypeError: Symfony\Component\Validator\Constraints\UniqueValidator::reduceElementKeys(): Argument#2 ($element) must be of type array, ...\ChildDTO given, called in .../UniqueValidator.php on line 48`If `$element = $normalizer($element);` is executed before `reduceElementKeys` it would enable using Assert\Unique with array of objects when correctly normalizedCommits-------77df90b959 [Validator] UniqueValidator - normalize before reducing keys