Using parent variables in extending class

Question

Hi,

I have three classes MainClass, Member, and Project. Member and Project extend the MainClass.

First I create a MainClass object, then a Member object and execute the function setMember:

$mainclass = new MainClass();
$member = new Member($id);
$mainclass->setMember($member);

When the $member variable is set to the current member I want to use this variable in my Project class but I can't get it to work :s

I need the member's id so in theory there are two possibilities:

mysql_query("INSERT INTO projects (title, user_id) VALUES('$title', ".$this->member->id.")");

or

mysql_query("INSERT INTO projects (title, user_id) VALUES('$title', ".$this->member->getId().")");

The first one, $this->member->id, results NULL

The second one, $this->member->getId(), gives me this error:

Call to a member function getId() on a non-object in ....

Here are my classes (I stripped most of it offcourse)

class MainClass{
    public $member = NULL;

    public function __construct(){}

    public function setMember($member)
    {
        $this->member = $member;
    }

    public function getMember()
    {
        return $this->member;
    }
}

class Project extends MainClass{
    public $id;

    public function __construct($id=NULL){
        $this->setId($id);
    }

    public function setId($id){
        $this->id = $id;
    }

    public function getId(){
        return $this->id;
    }

    public function addProject($title){
        mysql_query("INSERT INTO projects (title, user_id) VALUES('$title', ".$this->member->getId().")");
    }
}

class Member extends MainClass{
    public $id;

    public function __construct($id=NULL){
        $this->setId($id);
    }

    public function setId($id){
        $this->id = $id;
    }

    public function getId(){
        return $this->id;
    }       
}

Answer 1

Instead of creating an instance of MainClass, you should be creating an instance of Project.

$mainclass = new Project();
$member = new Member($id);
$mainClass->setMember($member);

Answer 2

You have two children extending a parent but the children classes can't "see" each other private or public. You need to import the data into the Project class.

Answer 3

Take a step back and look at the relationships between your classes. In your database, a Project "has a" Member(or 'user' in the DB).

Your class implementations do not reflect this relationship. MainClass "has a" Member, but Project does not. You're using a member of the base class to create this relationship.

Unless $member in MainClass is static, an instance of Project can't see what another instance of MainClass has set for it, because they're separate instances. I would not recommend making $member static, because you'd essentially be using it like a global variable.

Like I said, take a step back and look at the relationships. Do they generally follow the relationships in your database schema? Why do Member and Project derive from MainClass? What is their common behavior?

I realize I'm asking more questions than I'm answering, but my gut reaction is that this particular technical issue isn't your real problem-- it's that your class design needs to be rethunk a little bit.

Hope that helps.