Inheriting from Db class - Two databases connection | PHP

Question

In my app I used one database only, and I created class to provide some common methods. Now I need to implement additional database.

So instead of creating separate Db class to do same things, I want to call that Db class with db related parameters.

Problem is, when I try to extend Db class with a class where I provide methods to app, database connection isn't established - it is null.

Both Main and User are using different databases.

Class Main extends Db; Class User extends Db;

Why something like this doesn't work?

class Db {

    protected $link;
    private $host;
    private $user;
    private $pass;
    private $dbName;

    public function __construct($host, $user, $pass, $db) {
        $this->host = $host;
        $this->user = $user;
        $this->pass = $pass;
        $this->dbName = $db;
        $this->connect();
    }

public function connect() {

        try
        {

            $this->link = new PDO("mysql:host=" . $this->host .
                    ";dbname=" .  $this->dbName . ";charset=utf8mb4",  $this->user,  $this->pass );
            $this->link->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
            $this->link->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);

        } catch (PDOException $exc)
        {
            $this->setMessage($exc->getMessage());
        }
    }
}
$db = new Db(HOST, USER,PASS,DB)

class Main extends DB {

    public function __construct() {

    }
    /** App methods bellow*/
    }

$db2 = new Db(HOST2,USER2,PASS2,DB2);
class User extends DB {

    public function __construct() {

    }
    /** App methods bellow*/
    }

What would be easiest solution to implement two different database connections?

Thanks!

Edit:

After updating my code as suggested, I have issues with accessing connection property of DB, so I am not able to do queries in Main and User class.

What really confuses me, is the reason why that property it isn't visible to Main class?

Main class methods are depending heavily on db link, and I am not sure how to fix this..

Updated code:

class Main {

    protected $db;

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

public function showFreeSlotsPerPlan() {

        try
        {
            $SQL = "some query";

            $stmt = $this->db->prepare($SQL);
 /** Prepared statements bellow **/
    }
    }

class Db {

    protected $link;
    private $host;
    private $user;
    private $pass;
    private $dbName;

    public function __construct($host, $user, $pass, $db) {
        $this->host = $host;
        $this->user = $user;
        $this->pass = $pass;
        $this->dbName = $db;
        $this->connect();
    }
    }

And calling it as suggested:

$db = new Db(HOST, USER, PASS, DB);
$main = new Main($db);

And if I try to call a method f.e. showFreeSlotsPerPlan()

Fatal error: Call to undefined method Db::prepare() in /var/www/html/Master/class/Main.class.php

Then when I debug instance of Main I get the following:

Main Object
(
    [db:protected] => Db Object
        (
            [link:protected] => PDO Object
                (
                )

            [host:Db:private] => localhost
            [user:Db:private] => root
            [pass:Db:private] => password
            [dbName:Db:private] => dbname
        )

)

Edit2: Workaround is adding prepare method to the DB.php, and I was able to execute query successfully.

Still I am not sure is this best aproach. I admit that my design is not good at all :(

public function prepare($sqlQuery) {
    return $this->link->prepare($sqlQuery);
}

Why would your `Main` and `User` classes be extending from `DB`? Why not just pass the `DB` object into them as a parameter to the constructor and store it as a property? Surely you cannot make the statement that a User _is a_ Database, can you? — Jeff Lambert, Oct 19 '16 at 20:07
Using `public function __construct()` in child classes means that this constructor (which does nothing) will be used. — u_mulder, Oct 19 '16 at 20:07
If i don't use an empty constructon, I get the following: `Missing argument 1 for Db::__construct(), called in /var/www/html/Master/class/User.php on line 344 and defined in /var/www/html/Master/class/DB.php on line 19` Same for rest of 3 arguments On 344 line I have a instantiated object of User class: $user = new User(); — fugitive, Oct 19 '16 at 20:14

score 1 · Accepted Answer · edited May 23 '17 at 10:27

@u_mulder described the technical issue you are facing correctly. The parent Db constructor is not automatically called from a child constructor. You could try to get around this by calling the parent constructor explicitly:

class Main extends DB 
{

    public function __construct() 
    {
        parent::__construct();
    }
    // ...
}

The problem is that your parent (DB) requires parameters to it's constructor, so you would have to also pass those in to your child:

class Main extends DB 
{

    public function __construct($host, $user, $pass, $db) 
    {
        parent::__construct($host, $user, $pass, $db);
    }
    // ...
}

But this makes no sense. Your biggest problem is the design. Instead, you should favor composition of objects over inheritance, especially in this case. Try restructuring your class like this:

class Main 
{
    protected $db;

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

$mainDb = new Db(HOST, USER,PASS,DB)
$main = new Main($mainDb);

And do the same for your User class. When you do this, your protected $link property will no longer be visible inside of your Main class. This is Okay, because, really, your Main and User classes should not have visibility of that property because what is the point of having a Db class, anyway? You can get around this by adding some public methods to your Db class (e.g. public function query($sql, $parameters)) that are then called from your Main and User classes.

Once you get that working, I would suggest that you look into the Active Record Pattern, or look into scrapping your approach altogether in favor of incorporating one of the many open source ORMs that are already available.

Many thanks @Jeff Lambert for your idea and approach! I will try to apply this. Thanks again! — fugitive, Oct 19 '16 at 20:31

Inheriting from Db class - Two databases connection | PHP

Edit:

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