Change tracking using ext-mongodb's BSON API

output.log

$ php tracked_persistable.php 
object(MongoDB\Examples\User)#29 (3) {
  ["name"]=>
  string(7) "alcaeus"
  ["emails"]=>
  object(MongoDB\Examples\TrackedBSONArray)#24 (0) {
  }
  ["_id"]=>
  object(MongoDB\BSON\ObjectId)#15 (1) {
    ["oid"]=>
    string(24) "63637a94909ef8efcf01fc92"
  }
}
array(1) {
  ["$set"]=>
  array(2) {
    ["name"]=>
    string(7) "andreas"
    ["emails.2"]=>
    array(2) {
      ["type"]=>
      string(7) "private"
      ["address"]=>
      string(19) "private@example.com"
    }
  }
}
object(MongoDB\Examples\User)#32 (3) {
  ["name"]=>
  string(7) "andreas"
  ["emails"]=>
  object(MongoDB\Examples\TrackedBSONArray)#35 (0) {
  }
  ["_id"]=>
  object(MongoDB\BSON\ObjectId)#31 (1) {
    ["oid"]=>
    string(24) "63637a94909ef8efcf01fc92"
  }
}

tracked.php

<?php
declare(strict_types=1);

namespace MongoDB\Examples;

use MongoDB\BSON\ObjectId;
use MongoDB\BSON\Persistable;
use MongoDB\BSON\Serializable;
use MongoDB\BSON\Unserializable;
use MongoDB\Client;
use MongoDB\Model\BSONArray;
use ReflectionObject;
use ReflectionProperty;

use function array_filter;
use function array_key_exists;
use function count;
use function get_object_vars;
use function getenv;
use function var_dump;

require __DIR__ . '/../vendor/autoload.php';

#[Attribute(Attribute::TARGET_PROPERTY)]
class TrackChanges
{
}

interface Tracked extends Serializable, Unserializable
{
    public function getInitialData(): ?array;
}

trait TrackedTrait
{
    private readonly array $__initialData;

    public function getInitialData(): ?array
    {
        return $this->__initialData ?? null;
    }

    private function setInitialData(array $initialData): void
    {
        $this->__initialData = $initialData;
    }

    public function __debugInfo(): array
    {
        if ($this instanceof ArrayObject) {
            return $this->getArrayCopy();
        }

        $data = get_object_vars($this);
        unset($data['__initialData']);

        return $data;
    }
}

interface TrackedPersistable extends Tracked, Persistable
{
}

trait TrackedPersistableTrait
{
    use TrackedTrait;

    public function bsonSerialize(): array
    {
        $data = [];

        foreach (getTrackedProperties($this) as $rp) {
            if ($rp->getValue($this) === null) {
                continue;
            }

            $data[$rp->getName()] = $rp->getValue($this);
        }

        return $data;
    }

    public function bsonUnserialize(array $data): void
    {
        $initialData = [];

        foreach (getTrackedProperties($this) as $rp) {
            $key = $rp->getName();

            if (array_key_exists($key, $data)) {
                $rp->setValue($this, $data[$key]);
                $initialData[$key] = $data[$key];
            }
        }

        $this->setInitialData($initialData);
    }
}

class TrackedBSONArray extends BSONArray implements Tracked
{
    use TrackedTrait;

    public function bsonUnserialize(array $data): void
    {
        $this->setInitialData($data);

        parent::bsonUnserialize($data);
    }
}

/** @return array<ReflectionProperty> */
function getTrackedProperties(Tracked $ct): array
{
    $filter = ReflectionProperty::IS_PUBLIC | ReflectionProperty::IS_PROTECTED | ReflectionProperty::IS_PRIVATE;

    return array_filter(
        (new ReflectionObject($ct))->getProperties($filter),
        fn (ReflectionProperty $rp) => count($rp->getAttributes(TrackChanges::class)) > 0
    );
}

function getUpdate(TrackedPersistable $ctp): array
{
    return addChangesToUpdate($ctp);
}

function addChangesToUpdate(Tracked $ct, array &$update = [], string $prefix = ''): array
{
    $oldData = $ct->getInitialData();
    $newData = $ct->bsonSerialize();

    foreach ($newData as $key => $value) {
        if ($value instanceof Tracked && $value->getInitialData() !== null) {
            addChangesToUpdate($value, $update, "{$prefix}{$key}.");
            continue;
        }

        if ($value instanceof Tracked && $value->getInitialData() === null) {
            $value = $value->bsonSerialize();
        }

        // Set fields that are only present in $newData
        if (! isset($oldData[$key])) {
            $update['$set']["{$prefix}{$key}"] = $value;
            continue;
        }

        // TODO: Handle changed fields recursively
        if ($oldData[$key] != $newData[$key]) {
            $update['$set']["{$prefix}{$key}"] = $value;
        }
    }

    // Unset fields that are only present in $oldData
    foreach ($oldData as $key => $_) {
        if (! isset($newData[$key])) {
            $update['$unset']["{$prefix}{$key}"] = 1;
        }
    }

    return $update;
}

class Email implements TrackedPersistable
{
    use TrackedPersistableTrait;

    public function __construct(
        #[TrackChanges]
        public string $type,
        #[TrackChanges]
        public string $address
    ) {
    }
}

class User implements TrackedPersistable
{
    use TrackedPersistableTrait;

    public function __construct(
        #[TrackChanges]
        public string $name,
        #[TrackChanges]
        public TrackedBSONArray $emails = new TrackedBSONArray(),
        #[TrackChanges]
        public readonly ObjectId $_id = new ObjectId(),
    ) {
    }
}

$client = new Client(
    getenv('MONGODB_URI') ?: 'mongodb://127.0.0.1/',
    [],
    ['typeMap' => ['array' => TrackedBSONArray::class]]
);

$collection = $client->test->users;
$collection->drop();

$user = new User(name: 'alcaeus');
$user->emails[] = new Email('personal', 'personal@example.com');
$user->emails[] = new Email('work', 'work@example.com');

$collection->insertOne($user);

$trackedUser = $collection->findOne();
var_dump($trackedUser);

$trackedUser->name = 'andreas';
/* Note: this absolutely does not work for removing array elements. That will
 * likely require more active change tracking at the BSONArray level. */
$trackedUser->emails[] = new Email('private', 'private@example.com');

var_dump(getUpdate($trackedUser));

$collection->updateOne(['_id' => $trackedUser->_id], getUpdate($trackedUser));

var_dump($collection->findOne());