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

David Chelimsky dchelimsky at gmail.com
Sun Sep 28 14:35:22 EDT 2008


On Sun, Sep 28, 2008 at 1:18 PM, Michael Latta <lattam at mac.com> wrote:
> 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.

I think what you propose would be objectively more explicit, but
"cleanliness" is a very subjective thing.

This is tricky because AR is simultaneously violating the principle of
least surprise by giving you a proxy instead of the real collection
AND adhering to principles of encapsulation and duck-typing. The fact
that team.players is a proxy should not matter to its consumer one way
or the other as long as it can treat it just like the collection of
players.

Now, in the case of what rspec is doing in the change matcher, I would
say that rspec is getting bitten by the violation of the principle of
least surprise because the matcher is doing something that AR is not
expecting - further delaying evaluation of AR::A::AP's delayed
evaluation. But I think rspec can deal w/ that and alleviate the
burden on the rspec-rails user to know two different matchers and when
to use them.

I've already implemented this so it's all transparent for the user:
http://rspec.lighthouseapp.com/projects/5645-rspec/tickets/545.

I'm open to re-opening this but I'd like some feedback from other
people before I do. Anybody else have any opinions?

David




> 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
>
> _______________________________________________
> rspec-users mailing list
> rspec-users at rubyforge.org
> http://rubyforge.org/mailman/listinfo/rspec-users
>


More information about the rspec-users mailing list