On Dec 4, 2011, at 6:15 PM, Gordon wrote:
> hi, there
>
> I have been looking at
> https://www.relishapp.com/rspec/rspec-core/docs/helper-methods/define-helper-methods-in-a-module.
>
> This is the documentation for usage of extend and include.
>
> In the section, "Scenario: extend a module in only some example
> groups",
> I refer to the setup in RSpec.configure, "c.extend Helpers, :foo
> => :bar"
> implies that the methods in Helpers are available to examples with the
> metadata, ":foo=>:bar".
>
> ------------- Extract start ---------------
>
> describe "an example group with matching metadata", :foo => :bar do
> puts "In a matching group, help is #{help}"
>
> it "does not have access to the helper methods defined in the
> module" do
> expect { help }.to raise_error(NameError)
> end
> end
>
> ------------- Extract end ---------------
>
> Question: Why is the example expected to fail (ie.
> raise_error(NameError))
> when the meta data matches and it's expected to pass?
In Ruby, when we include modules in classes, their methods are available in
instances of those classes.
module Helper
def help
:available
end
end
class Foo
include Helper
end
foo = Foo.new
foo.help # => :available
We can also make the same methods available by extending an instance of a class.
class Bar
end
bar = Bar.new
bar.extend(Helper)
bar.help # => :available
Now the tricky part is that classes are objects as well - instances of the
Class class. This means that when you extend a class, its methods are available
as class methods, not as instance methods:
class Baz
extend Helper
end
Baz.help # => :available
Baz.new.help # => ERROR!!!!!!!
In RSpec, an example group is a class, whereas an example is (effectively) an
instance of that class. This is not 100% accurate, but let's go with that
metaphor for the moment. When you say "config.include SomeModule", it gets
included into example groups, making its methods available within examples
(instances of the group). When you say "config.extend SomeModule", it extends
the example group, making its methods available within at the group level, but
not within the examples.
In the scenario you cite, the module is used to extend the example group
matching :foo => :bar (the first group), so it is available at the group level,
as demonstrated by `puts "In a matching group, help is #{help}"` printing out
"In a matching group, help is available", but is not available in the other
group, as demonstrated by `puts "In a non-matching group, help is #{help rescue
'not available'}"` printing out "In a non-matching group, help is not
available", and it is not available to examples (instances) in either group, as
demonstrated by `expect { help }.to raise_error(NameError)` passing in both
examples.
HTH,
David
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