ApiPlatform\Core\Serializer\AbstractItemNormalizer::getAttributeValue PHP Method

    protected function getAttributeValue($object, $attribute, $format = null, array $context = [])
    {
        $propertyMetadata = $this->propertyMetadataFactory->create($context['resource_class'], $attribute, $this->getFactoryOptions($context));
        try {
            $attributeValue = $this->propertyAccessor->getValue($object, $attribute);
        } catch (NoSuchPropertyException $e) {
            if (null === $propertyMetadata->isChildInherited()) {
                throw $e;
            }
            $attributeValue = null;
        }
        $type = $propertyMetadata->getType();
        if ((is_array($attributeValue) || $attributeValue instanceof \Traversable) && $type && $type->isCollection() && ($collectionValueType = $type->getCollectionValueType()) && ($className = $collectionValueType->getClassName()) && $this->resourceClassResolver->isResourceClass($className)) {
            $value = [];
            foreach ($attributeValue as $index => $obj) {
                $value[$index] = $this->normalizeRelation($propertyMetadata, $obj, $className, $format, $context);
            }
            return $value;
        }
        if ($attributeValue && $type && ($className = $type->getClassName()) && $this->resourceClassResolver->isResourceClass($className)) {
            return $this->normalizeRelation($propertyMetadata, $attributeValue, $className, $format, $context);
        }
        return $this->serializer->normalize($attributeValue, $format, $context);
    }