ObjectToArrayMapper does not serialize single nested object properties

[radoslav-grencik]

Tempest version

3.18

PHP version

8.5

Operating system

Linux

Description

When mapping an object to an array or JSON, ObjectToArrayMapper serializes
top-level scalar/date properties correctly, and it also serializes arrays of
objects correctly. However, a single nested object property is left as an object
instead of being recursively mapped to an array.

This becomes visible when the nested object contains a DateTime property:
top-level dates are serialized using the configured date serializer, but dates
inside the single nested object are later JSON-encoded as the internal public
structure of the DateTime object.

Current behavior

Given an object like this:

use Tempest\DateTime\DateTime;
use Tempest\DateTime\FormatPattern;
use Tempest\Validation\Rules\HasDateTimeFormat;

final class ChildObject
{
    public function __construct(
        #[HasDateTimeFormat(FormatPattern::ISO8601)]
        public DateTime $createdAt,
    ) {}
}

final class ParentObject
{
    public function __construct(
        #[HasDateTimeFormat(FormatPattern::ISO8601)]
        public DateTime $createdAt,
        public ChildObject $child,
        /** @var ChildObject[] */
        public array $children,
    ) {}
}

And this mapper call:

$date = DateTime::parse('2026-08-19T12:34:56+00:00');

$array = map(new ParentObject(
    createdAt: $date,
    child: new ChildObject($date),
    children: [new ChildObject($date)],
))->toArray();

The resulting array serializes the top-level date and the array-of-objects date,
but leaves the single nested child object unmapped:

[
    'createdAt' => '2026-08-19T12:34:56.000Z',
    'child' => ChildObject { ... },
    'children' => [
        ['createdAt' => '2026-08-19T12:34:56.000Z'],
    ],
]

When using toJson(), the nested date is then encoded as the public structure of
the Tempest\DateTime\DateTime object, instead of as the configured date string:

{
  "createdAt": "2026-08-19T12:34:56.000Z",
  "child": {
    "createdAt": {
      "timezone": "UTC",
      "timestamp": {
        "seconds": 1787142896,
        "nanoseconds": 0
      },
      "year": 2026,
      "month": 8,
      "day": 19,
      "hours": 12,
      "minutes": 34,
      "seconds": 56,
      "nanoseconds": 0
    }
  },
  "children": [
    {
      "createdAt": "2026-08-19T12:34:56.000Z"
    }
  ]
}

Expected behavior

Single nested object properties should be recursively mapped in the same way as
objects inside array properties.

Expected toArray() output:

[
    'createdAt' => '2026-08-19T12:34:56.000Z',
    'child' => [
        'createdAt' => '2026-08-19T12:34:56.000Z',
    ],
    'children' => [
        ['createdAt' => '2026-08-19T12:34:56.000Z'],
    ],
]

Expected toJson() output should contain the nested date as a string as well:

{
  "createdAt": "2026-08-19T12:34:56.000Z",
  "child": {
    "createdAt": "2026-08-19T12:34:56.000Z"
  },
  "children": [
    {
      "createdAt": "2026-08-19T12:34:56.000Z"
    }
  ]
}

Possible cause

ObjectToArrayMapper::resolvePropertyValue() currently handles iterable object
properties recursively:

if ($property->getIterableType()?->isClass()) {
    foreach ($propertyValue as $key => $value) {
        if (! is_object($value)) {
            continue;
        }

        $propertyValue[$key] = map($value)->toArray();
    }

    return $propertyValue;
}

But single object properties do not seem to go through an equivalent recursive
object-to-array mapping step. If no serializer is selected for that property,
the object is returned as-is.

Suggested regression tests

This could be covered with tests in ObjectToArrayMapperTest and
ObjectToJsonMapperTest:

#[Test]
public function object_with_single_nested_object_to_array(): void
{
    $date = DateTime::parse('2026-08-19T12:34:56+00:00');

    $array = map(new ParentObject(
        createdAt: $date,
        child: new ChildObject($date),
        children: [new ChildObject($date)],
    ))->toArray();

    $this->assertSame(
        [
            'createdAt' => '2026-08-19T12:34:56.000Z',
            'child' => [
                'createdAt' => '2026-08-19T12:34:56.000Z',
            ],
            'children' => [
                ['createdAt' => '2026-08-19T12:34:56.000Z'],
            ],
        ],
        $array,
    );
}

If this behavior is considered unintended, I can send a focused PR with regression tests for ObjectToArrayMapper and ObjectToJsonMapper.