# Debugging a collection iterator
**URL:** https://discourse.cakephp.org/t/debugging-a-collection-iterator/8379
**Category:** Need Help
**Created:** [September 14, 2020, 5:24pm UTC](https://discourse.cakephp.org/t/debugging-a-collection-iterator/8379 "2020-09-14T17:24:59Z")
**Posts on this page:** 16
**Page:** 1
### Author:  [@Zuluru](https://discourse.cakephp.org/u/Zuluru)
#### Post date: [September 14, 2020, 5:24pm UTC](https://discourse.cakephp.org/t/debugging-a-collection-iterator/8379/1 "2020-09-14T17:24:59Z")
I have a collection (internally a FilterIterator), that starts with 3 elements and has a rejection that reduces that to 2. But running a `foreach` on it processes only a single element. I’m trying to debug this, but at a loss as to where I can put a useful breakpoint.
I’m using CakePHP 3.8.6, and PHPStorm. If I put a breakpoint on the `foreach` line and step into it, it goes into the `reject` function of the `CollectionTrait`, where it’s going to call my callback function, and the call stack indicates that this is called from the standard CallbackFilterIterator. Other breakpoints can find me inside the `__debugInfo` function.
The collection does know that it’s got 2 items in it, and if I add `toArray()` in the `foreach`, then it iterates over all the elements. (I am using this to work around this particular issue in the short term, but I don’t like not knowing what’s actually wrong, because it throws every other iteration on collections everywhere in my application into question.) Adding `->rewind()` before the `foreach` has no effect. Uses of the same collection elsewhere in the code seem just fine. (Note: Not strictly the same object, but a separate collection generated by the exact same code on the exact same input, so should be the same for all intents and purposes.)
This really should be rock-solid, I’d have thought. I can’t think of what might be breaking this. But even more frustrating, I can’t seem to debug what’s going wrong. ☹
---
### Author:  [@dreamingmind](https://discourse.cakephp.org/u/dreamingmind)
#### Post date: [September 14, 2020, 5:38pm UTC](https://discourse.cakephp.org/t/debugging-a-collection-iterator/8379/2 "2020-09-14T17:38:07Z")
Is it possible to put some dummy step into the process that you can break on?
```auto
$x = 1; //a meaning less statement that can serve as a break point
```
This ‘safe’ stepping stone in the river might allow you to then look around at the other values and figure out what is happening.
---
### Author:  [@Zuluru](https://discourse.cakephp.org/u/Zuluru)
#### Post date: [September 14, 2020, 5:51pm UTC](https://discourse.cakephp.org/t/debugging-a-collection-iterator/8379/3 "2020-09-14T17:51:23Z")
I can step through the contents of the `foreach` loop. All the variables are set as I expect for the first pass through. When it gets to the end of the loop, it simply doesn’t go back to the start of the loop, it carries on to whatever is after it.
---
### Author:  [@Zuluru](https://discourse.cakephp.org/u/Zuluru)
#### Post date: [September 14, 2020, 5:52pm UTC](https://discourse.cakephp.org/t/debugging-a-collection-iterator/8379/4 "2020-09-14T17:52:18Z")
Breaking inside the loop isn’t useful, in other words. I need to break somewhere in the iterator where it’s deciding whether there are more items to iterate over.
---
### Author:  [@dreamingmind](https://discourse.cakephp.org/u/dreamingmind)
#### Post date: [September 14, 2020, 7:32pm UTC](https://discourse.cakephp.org/t/debugging-a-collection-iterator/8379/5 "2020-09-14T19:32:51Z")
And there is no way to place the stepping-stone break in the iterator callback? Or is breaking in there not allowed at all?
Or that’s still not where you need to see?
---
### Author:  [@Zuluru](https://discourse.cakephp.org/u/Zuluru)
#### Post date: [September 14, 2020, 7:56pm UTC](https://discourse.cakephp.org/t/debugging-a-collection-iterator/8379/6 "2020-09-14T19:56:45Z")
Seems that the underlying iterator is a base PHP class which cannot be stepped into, like `substr`, etc.
---
### Author:  [@dreamingmind](https://discourse.cakephp.org/u/dreamingmind)
#### Post date: [September 14, 2020, 8:35pm UTC](https://discourse.cakephp.org/t/debugging-a-collection-iterator/8379/7 "2020-09-14T20:35:32Z")
Hmm…
Isn’t your FilterIterator a class you’ve written that extends php’s FileIterator? with your specific implementation of `accept()`?
An alternative would be to use the cake Collections-\>filter(). You should be able to step into the callable you provide to satisfy that… At least I know I routinely debug from inside those callables
---
### Author:  [@Zuluru](https://discourse.cakephp.org/u/Zuluru)
#### Post date: [September 14, 2020, 8:53pm UTC](https://discourse.cakephp.org/t/debugging-a-collection-iterator/8379/8 "2020-09-14T20:53:01Z")
It’s not my `FilterIterator` class, it’s Cake’s, which is the result of using the stock `filter` call. The data in the collection is, as far as I can tell, all correct, as is my callable. It’s the `next` function that I think would be instructive to step into, but Cake’s `FilterIterator` doesn’t re-implement that, so the actual implementation (from the PHP base class) is presumably written in C.
---
### Author:  [@raul338](https://discourse.cakephp.org/u/raul338)
#### Post date: [September 14, 2020, 9:49pm UTC](https://discourse.cakephp.org/t/debugging-a-collection-iterator/8379/9 "2020-09-14T21:49:55Z")
Im not sure if I fully understand what are you doing.
Cant you check printing with `debug()` on first line of the filter and last line (check `filtered`/`not filtered`) to see if all three items are being processed at least.
Can you share a reduced sample snippet to see any errors/bugs?
---
### Author:  [@Zuluru](https://discourse.cakephp.org/u/Zuluru)
#### Post date: [September 14, 2020, 10:05pm UTC](https://discourse.cakephp.org/t/debugging-a-collection-iterator/8379/10 "2020-09-14T22:05:34Z")
My filter function is being called. If I put a breakpoint in it, it is hit three times, and it correctly returns true for two of them. I have multiple other places where this exact code is working just fine (the collection is being created in an accessor on an entity). In this one place, I can very simply write:
```auto
foreach ($items as $item) {
$x = 1;
}
```
and when stepping through, the `$x = 1` line will only be hit once, while:
```auto
foreach ($items->toArray() as $item) {
$x = 1;
}
```
will hit it twice.
---
### Author:  [@raul338](https://discourse.cakephp.org/u/raul338)
#### Post date: [September 14, 2020, 10:33pm UTC](https://discourse.cakephp.org/t/debugging-a-collection-iterator/8379/11 "2020-09-14T22:33:29Z")
Does phpstorm have configured to show the values of the collection automatically? can you disable that?
It may have something like issue [#8438](https://github.com/cakephp/cakephp/issues/8438)
It may explain why it works with toArray instead of the collection
---
### Author:  [@Zuluru](https://discourse.cakephp.org/u/Zuluru)
#### Post date: [September 14, 2020, 10:54pm UTC](https://discourse.cakephp.org/t/debugging-a-collection-iterator/8379/12 "2020-09-14T22:54:09Z")
This also breaks if I’m _not_ debugging it, and PHPStorm isn’t involved at all. That’s why I started trying to debug it in the first place.
---
### Author:  [@dreamingmind](https://discourse.cakephp.org/u/dreamingmind)
#### Post date: [September 15, 2020, 3:29pm UTC](https://discourse.cakephp.org/t/debugging-a-collection-iterator/8379/13 "2020-09-15T15:29:10Z")
Well, it has to be the iterator, the data or the interaction of these two specific players…
Is it only one specific set of three particular elements that causes the problem. Can you send other sets through and have them work?
If it’s just these elements then we have to examine them for problems.
If nothing works in the filter… Can you write the code using a Reduce instead?
---
### Author:  [@Zuluru](https://discourse.cakephp.org/u/Zuluru)
#### Post date: [September 29, 2020, 6:37pm UTC](https://discourse.cakephp.org/t/debugging-a-collection-iterator/8379/14 "2020-09-29T18:37:31Z")
I’ve finally had some more time to track this down a bit more. There are specifics that I don’t think should be relevant to the situation, but it turns out are. The collection in question is returned by an accessor function on an entity. Let’s say I have `X hasMany Y`, so `$x->ys` is an array. `$x->special_ys` returns a collection of just the Y’s that I’m interested in, created using `->reject()`.
If I use `$x->special_ys`, then all is well in all cases.
If I assign `$ys = $x->special_ys` and then iterate over `$ys`, it gives me only the first one. But `$ys->count()` gives the right count, and `$ys->extract('field')` returns that field for each Y in the collection, and `$ys->toArray()` does return an array with all of the Ys in it.
I have tried changing my accessor to use `filter` with a reversed condition instead of `reject`, no change in behaviour.
I can certainly use `toArray` in this particular case. And it’s just generating a bit of output, so I can probably restructure it to use `extract` with a closure instead of my own `foreach` loop. But I still worry that there are more such uses lurking somewhere in these few hundred thousand lines of code…
---
### Author:  [@Zuluru](https://discourse.cakephp.org/u/Zuluru)
#### Post date: [September 29, 2020, 6:50pm UTC](https://discourse.cakephp.org/t/debugging-a-collection-iterator/8379/15 "2020-09-29T18:50:28Z")
Further detail, if I remove an earlier call to that accessor, then it works correctly in this place. So it’s perhaps something about the `unwrap` function of Cake’s `filterIterator` class. I will see about writing this up as an issue on GitHub.
---
### Author:  [@Zuluru](https://discourse.cakephp.org/u/Zuluru)
#### Post date: [September 29, 2020, 7:43pm UTC](https://discourse.cakephp.org/t/debugging-a-collection-iterator/8379/16 "2020-09-29T19:43:46Z")
I have narrowed it down still further. It’s something to do with inspecting the collection while iterating it. Stepping through in a debugger will trigger the erroneous behaviour, as will trying to count the elements. So some of what I wrote above may well not actually be true symptoms, but rather artifacts of me debugging it.
Here’s a very simple bit of code that will demonstrate the problem:
```auto
$x = collection([1, 2, 3, 4, 5])->reject(function ($i) { return $i > 3; });
foreach ($x as $i) {
$x->count();
pr($i);
}
```
For me, this generates just “1” as the output. If I remove the `count` call, it shows 1 2 3.
Maybe others on newer versions of Cake can double-check this? I am using 3.8.6 at the moment.