[rspec-users] Should change not comparing arrays how I expected

Michael Latta lattam at mac.com
Sun Sep 28 14:18:40 EDT 2008


David,

It seems to me that the root of the problem is that the specification  
is incorrect.  Since Rails returns association proxies the  
specification fails because it does not specify what the behavior  
should be.  I would suggest that instead of patching the change  
matcher, that you should add a change_contents matcher that matches  
the contents of a collection vs. the contents of a collection.  That  
way the framework is not guessing what was meant, but relying on the  
specification to be correct.  Since that is really what you want to  
specify (the contents have changed).  I think this is cleaner.

Michael


On Sep 28, 2008, at 11:13 AM, David Chelimsky wrote:

> On Sun, Sep 28, 2008 at 11:01 AM, David Chelimsky <dchelimsky at gmail.com 
> > wrote:
>> On Sun, Sep 28, 2008 at 10:43 AM, David Chelimsky <dchelimsky at gmail.com 
>> > wrote:
>>> On Sun, Sep 28, 2008 at 9:47 AM, Ashley Moran
>>> <ashley.moran at patchspace.co.uk> wrote:
>>>> Hi
>>>>
>>>> Just had a surprising result:
>>>>
>>>> it "should not appear in the Story.unposted list" do
>>>>   @story.save
>>>>   lambda {
>>>>     @story.post_to_twitter(@twitter_client)
>>>>   }.should change { Story.unposted }.from([@story]).to([])
>>>> end
>>>>
>>>> 'Story#post_to_twitter should not appear in the Story.unposted  
>>>> list' FAILED
>>>> result should have been changed to [], but is now []
>>>>
>>>> Anyone know why this fails?  I've looked in change.rb but I can't  
>>>> figure it
>>>> out.
>>>
>>> Whenever I've seen output like "should have been foo, but was foo"  
>>> it
>>> has boiled down to AR Assocation Proxies, which don't align in their
>>> response to == and inspect.
>>>
>>> I'm looking at seeing if there's a way we can make "should change"
>>> work in spite.
>>
>> Wow.
>>
>> OK - here's what I figured out. Talk about insidious bugs! This is
>> actually quite a bit different from what I thought.
>>
>> There are two lambdas involved here:
>>
>> lambda {
>> 1st lambda: expression that should cause the change
>> }.should change{
>> 2nd lambda: expression that returns the object that should change
>> }.to(expected value)
>>
>> The matcher executes the 1st lambda and stores the result as @before.
>> In your example this is a Rails association proxy for the
>> Story.unposted collection.
>>
>> The matcher then executes the 2nd lambda.
>>
>> The matcher then executes the 1st lambda again and stores the result
>> as @after. In your example, this is, again, a Rails association proxy
>> for the Story.unposted collection.
>>
>> At this point, @before and @after point to the same object - the same
>> Rails association proxy!!!!!!
>>
>> The matcher passes if @before != @after and fails if @before ==
>> @after. Because they are actually the same association proxy, the
>> example fails.
>>
>> Now rspec asks the matcher to print out the reason why it failed,
>> which does this:
>>
>> "#{result} should have been changed to #{@to.inspect}, but is now
>> #{@after.inspect}"
>>
>> @to is the expected value []
>> @after is the association proxy, which proxies to an empty  
>> collection.
>> Now, when the matcher calls @after.inspect, is the first time that  
>> the
>> proxy is actually evaluated!!!!
>>
>> Does this make sense?
>>
>> I was able to get a similar example to pass by doing this immediately
>> after storing the proxy in the @before variable:
>>
>> @before = @before.collect{|item|item} if @before.respond_to? 
>> (:collect)
>>
>> Ugly, ugly, ugly. But perhaps necessary to deal w/ this problem.
>>
>> I think I'll restructure things so the the change matcher handles  
>> this
>> in rails, but not in core rspec.
>>
>> Thoughts?
>
> FYI - ticket added and problem resolved:
> http://rspec.lighthouseapp.com/projects/5645-rspec/tickets/545
>
>>
>>>
>>>
>>>> I can make it work with:
>>>> should change { Story.unposted.length }.from(1).to(0)
>>>>
>>>> But that's a weaker test.
>>>>
>>>> Thanks
>>>> Ashley
>>>>
>>>> --
>>>> http://www.patchspace.co.uk/
>>>> http://aviewfromafar.net/
> _______________________________________________
> rspec-users mailing list
> rspec-users at rubyforge.org
> http://rubyforge.org/mailman/listinfo/rspec-users



More information about the rspec-users mailing list