[rspec-users] customize description and failure messages when writing rspec matcher dsl

David Chelimsky dchelimsky at gmail.com
Thu Nov 24 09:24:18 EST 2011


On Nov 24, 2011, at 12:48 AM, Anran Yang wrote:

> Dear all,
> 
> I recently tried to write a custom matcher using rspec dsl, which I'd like to use as the following:
> 
> ==========hpgc_spec.rb===================
> 
> require "spec_helper"
> 
> describe Hpgc do
>   specify do 
>     should be_structured_as(
>     { :app => {
>         :has => [
>             :name, 
>             :author, 
>             :org, 
>             :version, 
>             :abstract, 
>             :uri 
>         ],
> 
>         :can_be_a => { 
>             :dataset => { 
>                 :can_be_a => {
>                     :vector => {}, 
>                     :raster => {}
>                 }
>             },
> 
>             :program => { 
>                 :has => [
>                     :language, 
>                     :parallel_programming_model, 
>                     :parallel_performance_overview, 
>                     :category 
>                 ]
>             }
>         },
> 
>         :has_many => {
>             :app_option => {
>                 :has => [
>                     :switch, 
>                     :description 
>                 ]
>             },
> 
>             :app_io => { 
>                 :has => [
>                     :name, 
>                     :description, 
>                     :is_in 
>                 ]
>             }
>         }
>     }})
>   end
> end
> 
> ===========================================
> 
> The purpose is to test the entire model structure(attributes, inheritance and association).

Then term Behavior Driven Development came into being, in part, to discourage this approach to testing. We believe testing should be about behavior, and that testing structures leads to highly coupled tests that are hard to change.

I'd recommend thinking about the need for each of these attributes, come up with examples of how they are used and write those instead. That said, see below for more on how matchers work.

> Then I wrote following code to implement it:
> 
> ========be_structured_as.rb===================
> 
> # This is a matcher for test hpgc model structure
> 
> RSpec::Matchers.define :be_structured_as do |structure|
>   def check(namespace, klass, desc)
>     (desc[:has_many] ||= {}).keys.each { |item| klass.to_s.camelize.constantize.new.should have_many item.to_s.pluralize.to_sym }
>     if !desc[:can_be_a] || desc[:can_be_a].empty?
>       obj = Factory.create(klass)
>       obj.should have_these_attributes (desc[:has] ||= [])
>     else
>       desc[:can_be_a].keys.map { |sub| sub.to_s }.should == namespace.subclasses_of(klass.to_s)
>       desc[:can_be_a].each { |key, value| check namespace, key, value }
>     end
>   end
> 
>   match do |namespace|
>     self.class.send :include, namespace
>     structure.each { |key, value| check namespace, key, value }
>   end
> end
> 
> ===============================================
> 
> To simplify the matcher definition,  I used some other machers like "obj.should have_these_attributes (desc[:has] ||= [])", this works fine, thanks to the excellent new matcher dsl syntax. Howerver, I got trouble when running the test using "rspec spec/models/hpgc_spec.rb -fd". The test passed but print
> 
> <Screenshot.png>
> 
> instead of something like "Hpgc should be structured as ...". I think it may due to the other matcher I used because the message is from the matcher "have_these_attributes".

That's correct. The default behavior is that the block passed to the "match" method in the matcher DSL is expected to return a boolean. If it raises an exception instead, that exception bubbles up and becomes the source of the failure message.

> I try to add method "description" but seems it doesn't work.

"description" is used for the documentation formatter, not for failure messages. If you run "rspec spec --format documentation" you'll see names for all your examples.

> And the failure message is also a problem. I search the web and come across some solutions, but all of them need manually build the message. I've not come out a clean way to do this(I must add some invasive clauses to track the error) and think maybe the best behaviour would be that it "throw" the failure message of inner matchers or false-value expressions. But this time it just told me something like "Hpgc is expected to be structured as ...".

Instead of using the "match" method in the matcher, you can use "match_unless_raises", which returns false if it captures an exception, letting the matcher control the failure message (which is what I think you're looking for):

RSpec::Matchers.define :be_structured_as do |structure|
  def check(namespace, klass, desc)
    # ....
  end

  match_unless_raises do |namespace|
    self.class.send :include, namespace
    structure.each { |key, value| check namespace, key, value }
  end
end

HTH,
David

> Any idea?
> 
> Best Regards,
> 
> YANG Anran




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