[rspec-rails] For helper specs, why is the helper cached in the class between running each example?

Testing RSpec
1

I ran into a recent problem writing specs for a helper. I was testing
a helper that uses the standard memoization technique of caching the
result of an expensive calculation in an instance variable:

def something_expensive
@something_expensive ||= do_something_expensive
end

I have several different rspec examples for this one helper method,
all of which mock out a method that do_something_expensive calls,
which should in turn cause a different return value from
#something_expensive. When I ran my specs, I wound up getting the
same return value for each spec–the return value from the first spec
that ran. After investigating it a bit, I ran across this[1] code in
rspec-rails’ ExampleHelperGroup:

    def helper
      self.class.helper
    end

The #helper method simply delegates to the class’s helper method,
which memoizes the helper object in an instance variable. The result
of this is that the helper is cached in the class between example
runs, and because of the memoization in my helper method, subsequent
specs were returning the same value.

I found a work around:

after(:each) do
  helper.instance_variable_set('@something_expensive', nil)
end

But it feel like a bit of a hack, and it’s annoying/frustrating that I
have to do this. My specs shouldn’t have to be aware of the
memoization and manually clear it to work.

Why is the helper object cached in the class between running each
example? This can accidentally lead to spec interdependencies (i.e.
example B only passes if it runs after example A has run, because
example A puts the helper object into a certain state that example B
unknowingly depends on).

Thanks,
Myron

[1]

2

On Mon, Feb 8, 2010 at 1:44 PM, Myron M. [email protected]
wrote:

which should in turn cause a different return value from
which memoizes the helper object in an instance variable. The result
But it feel like a bit of a hack, and it’s annoying/frustrating that I
have to do this. My specs shouldn’t have to be aware of the
memoization and manually clear it to work.

Why is the helper object cached in the class between running each
example? This can accidentally lead to spec interdependencies (i.e.
example B only passes if it runs after example A has run, because
example A puts the helper object into a certain state that example B
unknowingly depends on).

Someone just submitted a ticket w/ a patch on this last week:

https://rspec.lighthouseapp.com/projects/5645-rspec/tickets/627

It’ll be released with rspec-rails-1.3.3, some time in the next few
days.

Cheers,
David

3

Thanks, David. I searched the google group for discussions of my
problem but forgot to search the github issues. I’ll be sure to check
there next time!

4

On Mon, Feb 8, 2010 at 9:31 PM, Myron M. [email protected]
wrote:

Thanks, David. I searched the google group for discussions of my
problem but forgot to search the github issues. I’ll be sure to check
there next time!

No worries - I was just surprised that this has been a theoretical
problem for so long and three people bring it up within a week of each
other! Glad we got it visible and fixed.

Cheers,
David