包含数据库查询的PHP构造函数构建对象，好/坏？

Question

Is it bad practice to have a database query in the constructor for a class in order to load it upon creating a new instance of it?

class Home
{
    private $home_id = null;
    private $home_name = null;
    private $home_number = null;
    private $home_street = null;

    function __construct($home_id)
    {
        $do_query = $mysql_con->query("SELECT * FROM home WHERE home_id = '$home_id'");

        while ($home_data = $do_query->fetch_assoc())
        {
            // Set all of the items in the object
            $this->home_id = $home_data["home_id"];
            $this->home_name = $home_data["home_name"];
            $this->home_number = $home_data["home_number"];
            $this->home_street = $home_data["home_street"];
        }
    }
}

I have been told before that this might be bad practice, to have a query that builds up the object in the constructor.

• If it is bad practice, why is it bad practice?
• What is the alternative?

Answer 1

Your Home class is a domain object and those should ideally not be aware of how they're persisted.

Separating the concerns allows for flexibility. This is also referred to as the Data Mapper pattern.

class Home
{
    public $home_id;
    public $home_name;
    public $home_number;
    public $home_street;
}

interface HomeMapperInterface
{
    public function get($id);
}

class HomeMapper implements HomeMapperInterface
{
    public function __construct($db)
    {
        $this->db = $db;
    }

    public function get($id)
    {
        $query = $this->db->query(...);
        if (($row = $do_query->fetch_assoc()) === false) {
            throw new RecordNotFoundException();
        }

        $home = new Home;
        $home->home_id = $row['home_id'];
        // ...

        return $home;
    }
}

To use it:

$mapper = new HomeMapper($db);
$home = $mapper->get(123);

You can improve this by using Identifier Map to avoid loading the same record twice into separate objects.

Btw, this is only a partial data mapper; it would also be used to update, insert and delete objects from your database.