[rspec-users] Parameterised shared examples / metadata in examples (RSpec 2)

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Ashley Moran

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Jul 23, 2010, 3:09:39 AM7/23/10
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Hi

Warning: this goes on quite a bit. It contains early-morning caffeinated ramblings and many "hmmm I wonder what this does..." snippets.

I'm looking for the best way to parameterise shared examples. Imagine (as an academic example...) you were doing it for subclasses of Struct instances (a more realistic example might be ActiveRecord subclasses, or DataMapper resources), such as:

class MyStruct < Struct.new(:a, :b)
end

class MyOtherStruct < Struct.new(:foo, :bar)
end


I've seen it done with #let, eg:

shared_examples_for "a Struct" do
it "has methods" do
properties.each do |property|
struct.should respond_to(property)
end
end
end

describe MyStruct do
let(:struct) { MyStruct.new }
let(:properties) { [:a, :b] }
it_should_behave_like "a Struct"
end

describe MyOtherStruct do
let(:struct) { MyOtherStruct.new }
let(:properties) { [:foo, :bar] }
it_should_behave_like "a Struct"
end

Which is not a bad solution, but does feel a bit too much like using (scoped) global variables for my liking. There's no explicit association between the shared examples and their parameters (and the arguments actually passed in each example group.


So I started to wonder if this could be done with metadata. My first naive stab was this:

describe MyStruct do
it_should_behave_like "a Struct", properties: [:a, :b]
end

But this fails:

Could not find shared example group named {:properties=>[:a, :b]}

Anyway, I dug in a bit and found that the metadata is only available to the example group anyway, not the examples themselves. So you can't do:

describe MyStruct, properties: [:a, :b] do
let(:struct) { MyStruct.new }
it "has methods" do
metadata[:properties].each do |property|
struct.should respond_to(property)
end
end
end

But (more digging), you can do this:

describe MyStruct, properties: [:a, :b] do
let(:struct) { MyStruct.new }
it "has methods" do
example.metadata[:properties].each do |property|
struct.should respond_to(property)
end
end
end

Which means I can get this close to my original dreamed-up syntax:

shared_examples_for "a Struct with metadata" do
it "has methods" do
example.metadata[:properties].each do |property|
struct.should respond_to(property)
end
end
end

describe MyStruct, properties: [:a, :b] do
let(:struct) { MyStruct.new }
it_should_behave_like "a Struct with metadata"
end

I don't object so much to having "struct" floating around, as it's fairly safe to say all the shared examples will depend on #struct being available. Although, arguably, #subject would be better:

shared_examples_for "a subject Struct with metadata" do
it "has methods" do
example.metadata[:properties].each do |property|
subject.should respond_to(property)
end
end
end

describe MyStruct, properties: [:a, :b] do
subject { MyStruct.new }
it_should_behave_like "a subject Struct with metadata"
end

or even:

shared_examples_for "a subject Struct with metadata" do
metadata[:properties].each do |property|
it { should respond_to(property) }
end
end

describe MyStruct, properties: [:a, :b] do
subject { MyStruct.new }
it_should_behave_like "a subject Struct with metadata"
end

I tried to be a bit clever to see if I could clean up the example definitions in the shared spec, but I got this far before hitting weirdness that was beyond my understanding of RSpec (and the reach of my spade...). But, this was a bit of a tangent anyway:

shared_examples_for "a subject Struct with metadata" do
metadata[:params].each { |key, value| define_method(key) { value } }

p self.inspect # outputs nil (!!!)

properties.each do |property|
it { should respond_to(property) }
end
end

describe MyStruct, params: {properties: [:a, :b]} do
subject { MyStruct.new }
it_should_behave_like "a subject Struct with metadata"
end


Sooooo... after all this, I just wondered if anyone had any ideas what the best way to achieve this is, and how it could be extended.

For example, would there be any merit in being able to write:

it_should_behave_like "a Struct", properties: [:a, :b]

?

Also I figure that as the metadata system is new, it's potentially unfinished and/or in flux. What are the plans/intentions/opportunities for expansion for it?

Cheers
Ash


--
http://www.patchspace.co.uk/
http://www.linkedin.com/in/ashleymoran

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Wincent Colaiuta

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Jul 23, 2010, 3:57:36 AM7/23/10
to rspec-users

Recently commited (RSpec 2.0.0.beta.18) was the ability to pass a block to "it_should_behave_like", making the relation clearer; eg:

describe MyStruct do
it_should_behave_like 'a Struct' do
let(:struct) { MyStruct.new }
end
end

I did ask about parametrizing that explicitly via metadata, but David feels that the block based approach is better; see the full thread here:

http://github.com/rspec/rspec-core/issues/71

Wincent

Ashley Moran

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Jul 23, 2010, 5:38:49 AM7/23/10
to rspec-users

On Jul 23, 2010, at 8:57 am, Wincent Colaiuta wrote:

> Recently commited (RSpec 2.0.0.beta.18) was the ability to pass a block to "it_should_behave_like", making the relation clearer; eg:
>
> describe MyStruct do
> it_should_behave_like 'a Struct' do
> let(:struct) { MyStruct.new }
> end
> end
>
> I did ask about parametrizing that explicitly via metadata, but David feels that the block based approach is better; see the full thread here:
>
> http://github.com/rspec/rspec-core/issues/71

And I only just tweeted about that ticket too! I didn't review it all though, I just saw the aliasing (which I currently do by hand).

That's in beta 18 then? Pretty sure I just got that when I updated this morning. Will play around with it later...

Cheers
Ash

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