Question

If I have a model that essentially represents a single row in a database, is it scale-friendly and test-friendly, and all around okay practice to have it populate it’s own properties, or should it have its properties injected, the same way you would inject an object?

Example:

Class blog {
    $id;
    $title;
    $body;

    public function load($id) {
        // db query to load id, title, body
    }
}

OR

Class blog {
    $id
    $title
    $body
}

// load blog data into $data, and then...

$blog = new Blog($data)

Answer 1

It’s considered a bad practice to commingle database access inside your model classes directly. Injecting values is generally preferred.

That said, if you were dead set on doing something like $model->load($id) and having it fetch from a datasource, you could get away with something like:

class Model {
    private $_dataProvider;

    // inject data-provider dependency in constructor
    public function __construct($dataProvider){
        $this->_dataProvider = $dataProvider;
    } 

    public function loadById($id){
        $myData = $this->_dataProvider->loadDataById($id);
        $this->setFoo($myData['foo']);
        ...
    } 
}

By injecting a data access class, you can pass a mock in for testing, or replace your database with some web service, or whatever. As long as $dataProvider has a loadDataById() method that takes an int and returns the appropriate data structure, you’re good.

Personally, I prefer keep my models nice and focused on representing whatever it is they’re modeling. I rely on external service classes and repositories to load data, inject it into models, and return them.