Can you help me better understand the object manager to avoid errors when using it?

Question

I'm working on learning Magento 2, and I'm having issues with the automatic constructor dependency injection in my resource model collection.

Here is my code:

namespace TylerSchade\Contacts\Model\ResourceModel\Contact;

use \Magento\Framework\Model\ResourceModel\Db\Collection\AbstractCollection;

class Collection extends AbstractCollection
{
    protected $session;

    public function __construct(
        \Magento\Framework\Data\Collection\EntityFactoryInterface $entityFactory,
        \Psr\Log\LoggerInterface $logger,
        \Magento\Framework\Data\Collection\Db\FetchStrategyInterface $fetchStrategy,
        \Magento\Framework\Event\ManagerInterface $eventManager,
        \Magento\Framework\Model\ResourceModel\Db\AbstractDb $resource,
        \Magento\Customer\Model\Session $session,
        \Magento\Framework\DB\Adapter\AdapterInterface $connection = null
    ) {
        $this->session = $session;

        parent::__construct($entityFactory, $logger, $fetchStrategy, $eventManager, $connection, $resource);
    }

    protected function _construct()
    {
        $this->_init('TylerSchade\Contacts\Model\Contact', 'TylerSchade\Contacts\Model\ResourceModel\Contact');
    }

    public function addCustomerFilter()
    {
        $this->addFieldToFilter('customer_id', $this->session->getCustomerId());
    }
}

When I run code that loads the collection, I receive the following error:

Fatal error: Uncaught Error: Cannot instantiate interface Magento\Framework\DB\Adapter\AdapterInterface in /var/www/magentotwo/magentotwo/vendor/magento/framework/ObjectManager/Factory/Dynamic/Developer.php on line 73

I've tried to reorder the parameters and specify the parameter that causes the error to a default of null, because I've seen that in core code, but that leads to more errors. I've also tried to add a di.xml file that specified the preference for the interface to Magento\Framework\DB\Adapter\Pdo\Mysql, thinking that perhaps the issue is that I haven't specified a preference for the interface, but then I receive the following errors:

2 exception(s):
Exception #0 (InvalidArgumentException): Configuration array must have a key for 'dbname' that names the database instance
Exception #1 (Zend_Db_Adapter_Exception): Configuration array must have a key for 'dbname' that names the database instance

How can I fix these errors? Also, can you help me better understand the object manager's constructor dependency injection to avoid errors like this in the future?

Answer 1

You should declare your parameters before optional of parent class.

Just pay attention on parent constructor:

public function __construct(
    \Magento\Framework\Data\Collection\EntityFactoryInterface $entityFactory,
    \Psr\Log\LoggerInterface $logger,
    \Magento\Framework\Data\Collection\Db\FetchStrategyInterface $fetchStrategy,
    \Magento\Framework\Event\ManagerInterface $eventManager,
    \Magento\Framework\DB\Adapter\AdapterInterface $connection = null,
    \Magento\Framework\Model\ResourceModel\Db\AbstractDb $resource = null
)

$resource parameter is optional, but you make it required.

Change you constructor to

public function __construct(
    \Magento\Framework\Data\Collection\EntityFactoryInterface $entityFactory,
    \Psr\Log\LoggerInterface $logger,
    \Magento\Framework\Data\Collection\Db\FetchStrategyInterface $fetchStrategy,
    \Magento\Framework\Event\ManagerInterface $eventManager,
    \Magento\Customer\Model\Session $session,
    \Magento\Framework\DB\Adapter\AdapterInterface $connection = null,
    \Magento\Framework\Model\ResourceModel\Db\AbstractDb $resource = null
)

should resolve your problem