rspec users - Feb 2010 - Applying an rspec matcher against the elements of a collection

Hi all,
I''ve tried to figure out whether rspec has any features to make it
easier to make assertions against the elements of a collection, but I
haven''t had any luck finding anything so far. I thought I''d
explain
the problem here, and propose a potential feature that might mitigate
it.

Let''s say I have a Person class:

class Person < Struct.new( :name, :age )

  VOTING_AGE = 18
  def self.get_voters( people )
    people.reject{ |person| person.age < VOTING_AGE }
  end

end

As you can see we have a method here which filters a collection of
people, returning only those people old enough to vote. If I were to
test this method in rspec I might write:

describe ''Person vote filtering'' do
	it ''filters out people younger than voting age'' do
		people = [
			Person.new( ''jenny'', 18 ),
			Person.new( ''dave'', 12 ),
			Person.new( ''paul'', 19 ),
			Person.new( ''lisa'', 17 )
		]

		voters = Person.get_voters( people )

		voter_names = voters.map{ |p| p.name }
		voter_names.should == [''jenny'',''paul'']
	end
end

This works, but having to manually pull out the voter names into a
seperate collection just in order to check who was filtered and who
wasn''t has always seemed clunky to me. What I would prefer is to be
able to check whether the collection contains person who matching my
expectations. Say I have a custom matcher:

Spec::Matchers.define :be_named do |expected|
	match do |actual|
		actual.name == expected
	end
end

Then I''d like to be able to write something like

voters.should( have(2).people )
voters.should( have_one_that( be_named(''jenny'') ) )
voters.should( have_one_that( be_named(''paul'') ) )

or even:

voters.should( have_elements_that(
	be_named( ''jenny'' ),
	be_named( ''paul'' )
)

To me this is a lot clearer - although the method names and how
they''re composed into the DSL could clearly use some work ;).

Now, for the trivial case I''ve been using as an example it would
probably be overkill, but I often find myself writing fairly
convuluted code at the end of a test just to figure out whether a
collection contains an element that matches some complex predicate. It
seems to me that if rspec had the generic ability to apply matchers to
the elements of a collection it would raise the level of
expressiveness for this kind of tests.

Thoughts? Does Rspec already support something like this that I''m just
not aware of? If I were to write a patch implementing this would it
have any chance of being accepted?

Cheers,
Pete

I''m going to argue that your design is off, and then ignore the rest of
your post :)

class Person < Struct.new(:name, :age)
  VOTING_AGE = 18

  def voter?
    age >= VOTING_AGE
  end
end

Now your tests become very simple:

Person.new(''Jenny'', 17).should_not be_voter
Person.new(''Bob'', 18).should be_voter

Why you want a Person.get_voters method to select voters from a list,
I''m not really sure.  You can always just do:

voters = collection_of_people.select {|p| p.voter?}

Also, RSpec has two mechanisms for testing collections the way you want (so I
guess I''m not ignoring your post after all).

If you only care about inclusion, you can use the include matcher:

jenny = Person.new(''Jenny'', 17)
bob = Person.new(''Bob'', 18)
sally = Person.new(''Sally'', 20)

voters = Person.get_voters(jenny, bob, sally)
voters.should include(bob, sally)
voters.should_not include(jenny)

there is also the set equality matcher, which checks that the contents of two
collections are equal irrespective of order:

voters.should =~ [bob, sally]

Pat

On Feb 23, 2010, at 10:18 AM, Pete Hodgson wrote:
> Hi all,
> I''ve tried to figure out whether rspec has any features to make it
> easier to make assertions against the elements of a collection, but I
> haven''t had any luck finding anything so far. I thought
I''d explain
> the problem here, and propose a potential feature that might mitigate
> it.
> 
> Let''s say I have a Person class:
> 
> class Person < Struct.new( :name, :age )
> 
>  VOTING_AGE = 18
>  def self.get_voters( people )
>    people.reject{ |person| person.age < VOTING_AGE }
>  end
> 
> end
> 
> As you can see we have a method here which filters a collection of
> people, returning only those people old enough to vote. If I were to
> test this method in rspec I might write:
> 
> describe ''Person vote filtering'' do
> 	it ''filters out people younger than voting age'' do
> 		people = [
> 			Person.new( ''jenny'', 18 ),
> 			Person.new( ''dave'', 12 ),
> 			Person.new( ''paul'', 19 ),
> 			Person.new( ''lisa'', 17 )
> 		]
> 
> 		voters = Person.get_voters( people )
> 
> 		voter_names = voters.map{ |p| p.name }
> 		voter_names.should ==
[''jenny'',''paul'']
> 	end
> end
> 
> This works, but having to manually pull out the voter names into a
> seperate collection just in order to check who was filtered and who
> wasn''t has always seemed clunky to me. What I would prefer is to
be
> able to check whether the collection contains person who matching my
> expectations. Say I have a custom matcher:
> 
> Spec::Matchers.define :be_named do |expected|
> 	match do |actual|
> 		actual.name == expected
> 	end
> end
> 
> Then I''d like to be able to write something like
> 
> voters.should( have(2).people )
> voters.should( have_one_that( be_named(''jenny'') ) )
> voters.should( have_one_that( be_named(''paul'') ) )
> 
> or even:
> 
> voters.should( have_elements_that(
> 	be_named( ''jenny'' ),
> 	be_named( ''paul'' )
> )
> 
> To me this is a lot clearer - although the method names and how
> they''re composed into the DSL could clearly use some work ;).
> 
> Now, for the trivial case I''ve been using as an example it would
> probably be overkill, but I often find myself writing fairly
> convuluted code at the end of a test just to figure out whether a
> collection contains an element that matches some complex predicate. It
> seems to me that if rspec had the generic ability to apply matchers to
> the elements of a collection it would raise the level of
> expressiveness for this kind of tests.
> 
> Thoughts? Does Rspec already support something like this that I''m
just
> not aware of? If I were to write a patch implementing this would it
> have any chance of being accepted?
> 
> Cheers,
> Pete
> _______________________________________________
> rspec-users mailing list
> rspec-users at rubyforge.org
> http://rubyforge.org/mailman/listinfo/rspec-users

Pat, Thanks for the response.

On Feb 23, 1:43?pm, Pat Maddox <mailingli... at patmaddox.com>
wrote:
> I''m going to argue that your design is off, and then ignore the
rest of your post :)
Fair enough :) The ''design'' in my example was made up on the
spot to
try and illustrate the kind of issues I''ve been coming up against
without lots of irrelevant detail. I agree that the code has all sorts
of silly issues.
>
> Also, RSpec has two mechanisms for testing collections the way you want (so
I guess I''m not ignoring your post after all).
>
Thanks for pointing me towards the include and =~ matchers. This was
part of what I was looking for, and would solve a lot of the issues
I''ve hit in the past.

That said, I still feel that it would be helpful to have some way of
applying a matcher against the elements of a collection. I will just
have to come up with a more plausible example... :)

Cheers,
Pete

On Tue, Feb 23, 2010 at 4:43 PM, Pat Maddox <mailinglists at
patmaddox.com> wrote:
<snip>
>
> there is also the set equality matcher, which checks that the contents of
two collections are equal irrespective of order:
>
> voters.should =~ [bob, sally]
>
I can''t believe I didn''t know =~ could be used for comparing
collections regardless of order, this is awesome!

Thanks,
Michael Guterl

I missed one other one that you''ll find handy.

voters.should have(2).items

Take a look at http://rspec.rubyforge.org/rspec/1.3.0/classes/Spec/Matchers.html
because there''s a lot of useful stuff...



On Feb 23, 2010, at 2:53 PM, Pete Hodgson wrote:
> Pat, Thanks for the response.
> 
> On Feb 23, 1:43 pm, Pat Maddox <mailingli... at patmaddox.com> wrote:
>> I''m going to argue that your design is off, and then ignore
the rest of your post :)
> 
> Fair enough :) The ''design'' in my example was made up on
the spot to
> try and illustrate the kind of issues I''ve been coming up against
> without lots of irrelevant detail. I agree that the code has all sorts
> of silly issues.
> 
>> 
>> Also, RSpec has two mechanisms for testing collections the way you want
(so I guess I''m not ignoring your post after all).
>> 
> 
> Thanks for pointing me towards the include and =~ matchers. This was
> part of what I was looking for, and would solve a lot of the issues
> I''ve hit in the past.
> 
> That said, I still feel that it would be helpful to have some way of
> applying a matcher against the elements of a collection. I will just
> have to come up with a more plausible example... :)
> 
> Cheers,
> Pete
> _______________________________________________
> rspec-users mailing list
> rspec-users at rubyforge.org
> http://rubyforge.org/mailman/listinfo/rspec-users