getFirstItem is not working on collection while getData returns an array

Question

getFirstItem is not working on a collection while getData returns a row.

$product_obj       = $objectManager->create('Magento\Catalog\Model\Product');
$productCollection = $product_obj->getCollection()->addAttributeToFilter('erp_vc_item_code',$value->getVcItemCode());

print_r($productCollection->getSize()); // Prints 1
print_r($productCollection->getData()); // Prints an array
print_r($productCollection->getFirstItem()->getData()); // Does not prints anything
print_r($productCollection->getFirstItem()->getSku()); // Does not prints anything

Output

1

Array
(
  [0] => Array
    (
        [entity_id] => 2087
        [attribute_set_id] => 4
        [type_id] => simple
        [sku] => Mouthwash---Fresh-To-Go-500ml
        [has_options] => 0
        [required_options] => 0
        [created_at] => 2022-03-27 16:31:08
        [updated_at] => 2022-03-27 16:31:09
        [erp_vc_item_code] => PCMO0000258
    )

)

Array
(
)

Sometimes the function works at other places but don't know why it does't work every time as expected.

Answer 1

Methods getSize, getData and getFirstItem use different logic

getSize

Calculates totals based on COUNT query

    /**
     * Get collection size
     *
     * @return int
     */
    public function getSize()
    {
        if ($this->_totalRecords === null) {
            $sql = $this->getSelectCountSql();
            $this->_totalRecords = $this->_totalRecords ?? $this->getConnection()->fetchOne($sql, $this->_bindParams);
        }
        return (int)$this->_totalRecords;
    }

getData

Loads items data without convert them into objects

    /**
     * Get all data array for collection
     *
     * @return array
     */
    public function getData()
    {
        if ($this->_data === null) {
            $this->_renderFilters()->_renderOrders()->_renderLimit();
            $select = $this->getSelect();
            $this->_data = $this->_fetchAll($select);
            $this->_afterLoadData();
        }
        return $this->_data;
    }

getFirstItem

Try to load collection and returns first element. Returns empty entity if items is empty

    /**
     * Retrieve collection first item
     *
     * @return DataObject
     */
    public function getFirstItem()
    {
        $this->load();

        if (count($this->_items)) {
            reset($this->_items);
            return current($this->_items);
        }

        return $this->_entityFactory->create($this->_itemObjectClass);
    }

For debug your case I suggest to do:

1. Run $productCollection->load(true); and check query

2. Print count($productCollection->getItems()); - maybe you have empty items

Answer 2

Try this:

<?php
     $objectManager = \Magento\Framework\App\ObjectManager::getInstance(); 
     $productCollection = $objectManager->create('Magento\Catalog\Model\ResourceModel\Product\Collection');
     $collection = $productCollection->addAttributeToSelect('*')->setPageSize(10);
     print_r($collection->getFirstItem()->getData();
?>