[ruby-core:44853] Re: help understanding weird bug

Thank you Aaron and Jeremy.

Basically it is a plain array which is modified by some hook when 
setting an association.

Now that I know that, the fix is obvious. I was just trying to figure 
out how to debug such kind of code in Ruby to understand where to look 
for, I mean, which code was changing the array.

Whether this is a good design choice, I'll discuss in the sequel-talk list.

Thanks,
Rodrigo.

Em 03-05-2012 12:35, Jeremy Evans escreveu:
> On 05/03 08:56, Rodrigo Rosenfeld Rosas wrote:
>> Here is a sample application:
>>
>> https://github.com/rosenfeld/weird-sequel-bug
>>
>> And here is the strange behavior:
>>
>> https://github.com/rosenfeld/weird-sequel-bug/blob/master/example.rb
>>
>> children = Sq::Node[1].children
>> p children.class
>> parent = children.first.parent
>> p children.map&:id
>> 2.times { p children.map{|c| c.parent = parent; c.id} }
>>
>> Output:
>>
>> Array
>> [2, 3]
>> [2, 2]
>> [3, 3]
>>
>> For some strange reason, changing the parent of the element inside
>> children.map{} block will modify the children array itself.
>>
>> for example:
>>
>> p children.map&:id
>> children[0].parent = parent
>> p children.map&:id
>>
>> outputs:
>> [2, 3]
>> [3, 2]
>>
>> Maybe this is a Sequel bug (I didn't look for the code of
>> associations yet), but I'd like to understand how this could happen
>> in Ruby.
>
> This isn't a Sequel bug or a ruby bug.  When you use c.parent= in
> Sequel, it will recognize that parent.children is the reciprocal, and
> c.parent= will modify parent.children.
>
> You issue is that you are modifying an array while you are iterating
> over it, which is generally a bad idea.
>
> Now, I can probably change it so that it doesn't make such a change
> if you pass in a object that is the same as the existing value, but I
> don't consider the current implementation incorrect.
>
>> For example, how can I know if this is an ordinary Array or not?
>> children.class is Array, so how can I differentiate it from a normal
>> Array.
> Sequel uses a plain array, it doesn't use a proxy object for
> associations.  You could probably use an after_load hook to extend the
> array with a module, though I'm not sure what it is you want to do.
>
>> If I change the first line to the one below it works:
>> # children = Sq::Node[1].children
>> children = Sq::Node[1].children.map{|c| c}
> Here you are creating a duplicate of the array, similar to:
>
>    children = Sq::Node[1].children.dup
>
> So you are not modifying the array you are iterating over, and hence,
> you don't have that problem.
>
> Jeremy
>