Making CakePHP’s TreeBehavior work with scope

We’re not talking mouthwash here. We’re talking code.

CakePHP’s TreeBehavior is cool, but tree traversal is a pretty arcane concept, even for developers, and MPTT is not something that is easy to digest mentally, or to develop around. Unfortunately, it’s also not incredibly efficient on large data sets. Even not-so-large data sets, on the order of a few hundred items, make certain actions — reordering, in particular — really processor intensive.

I’m using a JavaScript drag-and-drop tool in cms34 to allow admins to manage the page tree on their sites, and the data gets stored using CakePHP’s TreeBehavior, which is MPTT-based. The problem is, it really wasn’t working. So I completely rebuilt the code that assigns the über-critical “left” and “right” values to each node in the tree. My new way is much faster, even though it may break a few rules, and requires bypassing TreeBehavior’s callbacks.

Once I got that working, I was delighted, but then I discovered some other problems, namely pertaining to… scope. My CMS supports multiple sites in one installation, which means multiple trees, which means scope. The problem is, I was having a hell of a time figuring out just how to make scope work with TreeBehavior. Finally I found a link to a succinct and effective solution.

The upshot here is that you can’t define your $actsAs in the model, because… well… to be honest, I only have a vague understanding of why not, but essentially it’s due to the split roles of the model-view-controller framework. You’re building a rule that requires access to specific data values, which is something that needs to happen in the controller; the model is strictly for the abstract structure of the data. I understand it just enough to agree that it makes sense not to do it in the model. Which means you have to do it in the controller. The sample code from the link above goes a little something like this:

function add() {
    $this->Task->Behaviors->attach('Tree', array(
         'scope' => "Task.schedule_id = {$this->data['Task']['schedule_id']}"

That didn’t quite work in my situation, but it got me far enough along that I could figure out what to do from there.

I still need to do a little more testing to make sure my solution to the more efficient tree reordering is rock solid, and then I’ll post a tutorial. But for now, I hope this helps spread the word that scope does work on TreeBehavior… if you do it right.

Update (September 22, 2010): Although this didn’t really seem to be breaking anything, just throwing up a warning (when debugging was turned on), I discovered a minor issue with this code yesterday. Turns out CakePHP expects the value of scope to be an array. Just taking the string it was defined as and wrapping it in array() did the trick.