How to Avoid 'Call to a member function query() on a non-object' Errors Using Dependency Injection?

Avoiding Global Variables in Class Methods: A Pagination Example

When building pagination functionality, you might encounter the error "Call to a member function query() on a non-object." This often stems from accessing variables declared outside a class from within the class itself. Let's explore better solutions.

Dependency Injection: The Preferred Approach

Dependency injection provides a clean solution. Instead of relying on global variables, we pass the necessary objects (like the database connection) directly to the class. This is done through the constructor:

<code class="language-php">$db = new DB_MySQL("localhost", "root", "", "test"); // Database connection
include_once("pagi.php");

$pagination = new Paginator($db);
$records = $pagination->get_records("SELECT the, fields, you, want, to retrieve FROM `table`");

class Paginator {
    protected $db;

    public function __construct(DB_MySQL $db) {
        $this->db = $db;
    }

    public function get_records($q) {
        $x = $this->db->query($q);
        return $this->db->fetch($x);
    }
}</code>

Method Injection: An Alternative

Alternatively, you can inject the database object directly into the method needing it:

<code class="language-php">$db = new DB_MySQL("localhost", "root", "", "test"); // Database connection
include_once("pagi.php");

$pagination = new Paginator();
$records = $pagination->get_records("SELECT the, fields, you, want, to retrieve FROM `table`", $db);

class Paginator {
    public function get_records($q, DB_MySQL $db) {
        $x = $db->query($q);
        return $db->fetch($x);
    }
}</code>

Why Dependency Injection is Superior

Dependency injection offers significant advantages over using global variables:

• Explicit Dependencies: The code clearly shows what each class depends on, improving readability and maintainability.
• Loose Coupling: Classes are not tightly bound to specific implementations. You can easily swap database systems or other dependencies without altering core class logic.
• Testability: Unit testing becomes much simpler. You can easily mock dependencies during testing, isolating the class under test.

In summary, dependency injection is a best practice for managing dependencies and shared resources, promoting cleaner, more robust, and testable code. Avoid the pitfalls of global variables by adopting this approach.

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