modelling many-to-many objects?

pdaoust

I hope this is the right board to post on, and I hope I'm not duping here. Apologies if I'm transgressing, but here goes!

I'm trying to model objects with many-to-many relationships. One object is 'group', and one object is 'user'. Just like in the UNIX world, one user can belong to many groups, and one group can contain many users. Modelling that in my database was easy; I just created a junction table. But I came up against a rock wall when I started to model them in PHP:

class User {
    var $id, $username, $password;
    var $groups = array ();

function User () {
}

function create ($username) {
    global $conn;
    $recordSet = &$conn->Execute('SELECT * FROM users WHERE username = ' . $username);
    $this->username = $recordSet->fields['username'];
    $this->password = $recordSet->fields['password'];
    $recordSet = &$conn->Execute('SELECT g.groupname AS groupname FROM usersToGroups ug, groups g WHERE ug.groupID = g.id');
    $i = 0;
    while (!$recordSet->EOF) {
        $this->groups[$i] = new Group;
        $this->groups[$i]->create ($recordSet->fields['groupname'];
        $recordSet->MoveNext();
        $i ++;
    }
}
}

class Group {
    var $id, $groupname;
    var $users = array ();

function Group () {
}

function create ($groupname) {
    global $conn;
    $recordSet = &$conn->Execute('SELECT * FROM groups WHERE groupname = ' . $groupname);
    $this->groupname = $recordSet->fields['groupname'];
    $recordSet = &$conn->Execute('SELECT u.username AS username FROM usersToGroups ug, users u WHERE ug.userID = u.id');
    $i = 0;
    while (!$recordSet->EOF) {
        $this->users[$i] = new User;
        $this->users[$i]->create ($recordSet->fields['username'];
        $recordSet->MoveNext();
        $i ++;
    }
}
}

(these two classes are dramatically simplified from the originals, just for the sake of brevity. Also of note is that I'm using ADODB; $conn is an already instantiated ADODB class.)

Sooooooo... the problem here is that using this set of data:

user 'steve' belongs to groups 'admin', 'users', 'editors'
group 'editors' contains users 'rachel', 'steve', 'pete'

Instantiating a User using create('steve') will grab Steve's groups and instantiate them as Group objects, including the 'editors' group, which will in turn end up instantiating a User named 'steve', who will instantiate a Group named 'editors', and so on ad infinitum.

Any comments or suggestions on how I could best model this? Right now User->groups is just an array containing strings instead of Group objects, and Group->users is the same. That way, new objects can be instantiated at will without creating a cascade of new objects. But it seems to me that there should be a more 'OOP pure' way of doing this...

Thanks in advance!
Paul d'Aoust

tsinka

Hi,

here's an example that worked for me. Notice that you can add a user to a group and then change the name of a user in the user object. That updated name will be available if you get the user object from the group object since the add functions use call by reference.

It might not be the best solutions and looks like some sort of a hack. I don't know if it works with PHP 5 but my tests with a recent PHP 4 version were successful.

It's just an example that lacks any error checking or something like that.

<?PHP
class user {
  var $groups;
  var $id;
  var $name;

  function user($id,$name,$groups=array()) {
    $this->groups   = array();
    $this->name     = $name;
    $this->id       = $id;
  }

  function getGroup($id) {
    return $this->groups[$id];
  }

  function getGroups() {
    return $this->groups;
  }

  function addGroup(&$group) {
    $this->groups[$group->getId()]   = &$group;
  }

  function getName() {
    return $this->name;
  }

  function setName($name) {
    $this->name = $name;
  }

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

class group {
  var $users;
  var $id;
  var $name;

  function group($id, $name,$users=array()) {
    $this->id   = $id;
    $this->name = $name;
  }

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

  function getName() {
    return $this->name;
  }

  function setName($name) {
    $this->name = $name;
  }

  function getUsers() {
    return $this->users;
  }

  function getUser($id) {
    if (array_key_exists($id,$this->users)) {
      return $this->users[$id];
    } else {
      return false;
    }
  }

  function addUser(&$user) {
    $this->users[$user->getId()]   = &$user;
  }
}

$group1 = new group(1,"Group 1");
$group2 = new group(2,"Group 2");
$user1  = new user(1,"User 1");
$user2  = new user(2,"User 2");

// add user
$group1->addUser($user1);

// now change username
$user1->setName("User1b");

// get user stored in group 1
$user = $group1->getUser(1);

// the updated username will be displayed
// although we didn't update the name of
// the user object we stored in the group.
echo $user->getName();

$user1->addGroup($group1);
$group1->setName("MyName");
$group = $user1->getGroup(1);
echo $group->getName();
$user1->addGroup($group2);
print_r($user1->getGroups());
?>

Thomas

pdaoust

oh, I see, so your Group object doesn't instantiate User objects for each of it members; it relies on you to instantiate each of the members. Is that true? I never thought of that... hmmmmm, interesting...

Sgarissta

Originally posted by pdaoust
oh, I see, so your Group object doesn't instantiate User objects for each of it members; it relies on you to instantiate each of the members. Is that true? I never thought of that... hmmmmm, interesting...

From a resources standpoint this is far more effective, as in reality what are the odds of you NEEDING all user data, on all users in a given group, when the group was instantiated by a single user object 😃

pdaoust

Originally posted by Sgarissta
From a resources standpoint this is far more effective, as in reality what are the odds of you NEEDING all user data, on all users in a given group, when the group was instantiated by a single user object 😃

hmmm, there is wisdom in what you say. I think that each Group object is simply going to contain an array of usernames rather than User objects, and vice versa for the User object. And then I'll just instantiate each object as needed.

Thanks for the help, guys!