Which way is better for db handler?

Question

Is it a bad idea for a model class hold the db handler as a property? Example code are as below. I think the second is better, but the first is not so bad in my opinion.

First:

class ActiveModel {
    protected $db;

    public function __construct() {
        $this->db = DbFactory::get('Foo');
    }

    public function save() {
        //use $this->db to do the job;
        ....
    }
}

Second:

class ActiveModel {

    public function save($db = null) {
        if ($db === null) {
            $db = DbFactory::get('Foo');
        }
        //then use $db to do the job;
        ....
    }
}

Answer 1

It is fine and very convenient to keep the database handler as a property. However, you should pass it in the constructor via dependency injection, rather than call it's factory method inside the class. This can ease unit testing when you need to mock a database connection object.

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