Testing modules in RSpec

Question

What are the best practices on testing modules in RSpec? I have some modules that get included in few models and for now I simply have duplicate tests for each model (with few differences). Is there a way to DRY it up?

Answer 1

The rad way =>

let(:dummy_class) { Class.new { include ModuleToBeTested } }

Alternatively you can extend the test class with your module:

let(:dummy_class) { Class.new { extend ModuleToBeTested } }

Using 'let' is better than using an instance variable to define the dummy class in the before(:each)

When to use RSpec let()?

Answer 2

What mike said. Here's a trivial example:

module code...

module Say
  def hello
    "hello"
  end
end

spec fragment...

class DummyClass
end

before(:each) do
  @dummy_class = DummyClass.new
  @dummy_class.extend(Say)
end

it "get hello string" do
  expect(@dummy_class.hello).to eq "hello"
end

Answer 3

For modules that can be tested in isolation or by mocking the class, I like something along the lines of:

module:

module MyModule
  def hallo
    "hallo"
  end
end

spec:

describe MyModule do
  include MyModule

  it { hallo.should == "hallo" }
end

It might seem wrong to hijack nested example groups, but I like the terseness. Any thoughts?

Answer 4

I found a better solution in rspec homepage. Apparently it supports shared example groups. From https://www.relishapp.com/rspec/rspec-core/v/2-13/docs/example-groups/shared-examples!

Shared Example Groups

You can create shared example groups and include those groups into other groups.

Suppose you have some behavior that applies to all editions of your product, both large and small.

First, factor out the “shared” behavior:

shared_examples_for "all editions" do   
  it "should behave like all editions" do   
  end 
end

then when you need define the behavior for the Large and Small editions, reference the shared behavior using the it_should_behave_like() method.

describe "SmallEdition" do  
  it_should_behave_like "all editions"
  it "should also behave like a small edition" do   
  end 
end

Answer 5

Off the top of my head, could you create a dummy class in your test script and include the module into that? Then test that the dummy class has the behaviour in the way you'd expect.

EDIT: If, as pointed out in the comments, the module expects some behaviours to be present in the class into which it's mixed, then I'd try to implement dummies of those behaviours. Just enough to make the module happy to perform its duties.

That said, I'd be a little nervous about my design when a module expects a whole lot from its host (do we say "host"?) class - If I don't already inherit from a base class or can't inject the new functionality into the inheritance tree then I think I'd be trying to minimise any such expectations that a module might have. My concern being that my design would start to develop some areas of unpleasant inflexibility.

Answer 6

The accepted answer is the right answer I think, however I wanted to add an example how to use rpsecs shared_examples_for and it_behaves_like methods. I mention few tricks in the code snippet but for more info see this relishapp-rspec-guide.

With this you can test your module in any of the classes which include it. So you really are testing what you use in your application.

Let's see an example:

# Lets assume a Movable module
module Movable
  def self.movable_class?
    true
  end

  def has_feets?
    true
  end
end

# Include Movable into Person and Animal
class Person < ActiveRecord::Base
  include Movable
end

class Animal < ActiveRecord::Base
  include Movable
end

Now lets create spec for our module: movable_spec.rb

shared_examples_for Movable do
  context 'with an instance' do
    before(:each) do
      # described_class points on the class, if you need an instance of it: 
      @obj = described_class.new

      # or you can use a parameter see below Animal test
      @obj = obj if obj.present?
    end

    it 'should have feets' do
      @obj.has_feets?.should be_true
    end
  end

  context 'class methods' do
    it 'should be a movable class' do
      described_class.movable_class?.should be_true
    end
  end
end

# Now list every model in your app to test them properly

describe Person do
  it_behaves_like Movable
end

describe Animal do
  it_behaves_like Movable do
    let(:obj) { Animal.new({ :name => 'capybara' }) }
  end
end

Answer 7

my recent work, using as little hard-wiring as possible

require 'spec_helper'

describe Module::UnderTest do
  subject {Object.new.extend(described_class)}

  context '.module_method' do
    it {is_expected.to respond_to(:module_method)}
    # etc etc
  end
end

I wish

subject {Class.new{include described_class}.new}

worked, but it doesn't (as at Ruby MRI 2.2.3 and RSpec::Core 3.3.0)

Failure/Error: subject {Class.new{include described_class}.new}
  NameError:
    undefined local variable or method `described_class' for #<Class:0x000000063a6708>

Obviously described_class isn't visible in that scope.

Answer 8

To test your module, use:

describe MyCoolModule do
  subject(:my_instance) { Class.new.extend(described_class) }

  # examples
end

To DRY up some things you use across multiple specs, you can use a shared context:

RSpec.shared_context 'some shared context' do
  let(:reused_thing)       { create :the_thing }
  let(:reused_other_thing) { create :the_thing }

  shared_examples_for 'the stuff' do
    it { ... }
    it { ... }
  end
end

require 'some_shared_context'

describe MyCoolClass do
  include_context 'some shared context'

  it_behaves_like 'the stuff'

  it_behaves_like 'the stuff' do
    let(:reused_thing) { create :overrides_the_thing_in_shared_context }
  end
end

Resources:

• Shared Examples
• Shared Context
• BetterSpecs

Answer 9

What about:

describe MyModule do
  subject { Object.new.extend(MyModule) }
  it "does stuff" do
    expect(subject.does_stuff?).to be_true
  end
end

Answer 10

I would suggest that for larger and much used modules one should opt for the "Shared Example Groups" as suggested by @Andrius here. For simple stuff for which you don't want to go through the trouble of having multiple files etc. here's how to ensure maximum control over the visibility of your dummy stuff (tested with rspec 2.14.6, just copy and paste the code into a spec file and run it):

module YourCoolModule
  def your_cool_module_method
  end
end

describe YourCoolModule do
  context "cntxt1" do
    let(:dummy_class) do
      Class.new do
        include YourCoolModule

        #Say, how your module works might depend on the return value of to_s for
        #the extending instances and you want to test this. You could of course
        #just mock/stub, but since you so conveniently have the class def here
        #you might be tempted to use it?
        def to_s
          "dummy"
        end

        #In case your module would happen to depend on the class having a name
        #you can simulate that behaviour easily.
        def self.name
          "DummyClass"
        end
      end
    end

    context "instances" do
      subject { dummy_class.new }

      it { subject.should be_an_instance_of(dummy_class) }
      it { should respond_to(:your_cool_module_method)}
      it { should be_a(YourCoolModule) }
      its (:to_s) { should eq("dummy") }
    end

    context "classes" do
      subject { dummy_class }
      it { should be_an_instance_of(Class) }
      it { defined?(DummyClass).should be_nil }
      its (:name) { should eq("DummyClass") }
    end
  end

  context "cntxt2" do
    it "should not be possible to access let methods from anohter context" do
      defined?(dummy_class).should be_nil
    end
  end

  it "should not be possible to access let methods from a child context" do
    defined?(dummy_class).should be_nil
  end
end

#You could also try to benefit from implicit subject using the descbie
#method in conjunction with local variables. You may want to scope your local
#variables. You can't use context here, because that can only be done inside
#a describe block, however you can use Porc.new and call it immediately or a
#describe blocks inside a describe block.

#Proc.new do
describe "YourCoolModule" do #But you mustn't refer to the module by the
  #constant itself, because if you do, it seems you can't reset what your
  #describing in inner scopes, so don't forget the quotes.
  dummy_class = Class.new { include YourCoolModule }
  #Now we can benefit from the implicit subject (being an instance of the
  #class whenever we are describing a class) and just..
  describe dummy_class do
    it { should respond_to(:your_cool_module_method) }
    it { should_not be_an_instance_of(Class) }
    it { should be_an_instance_of(dummy_class) }
    it { should be_a(YourCoolModule) }
  end
  describe Object do
    it { should_not respond_to(:your_cool_module_method) }
    it { should_not be_an_instance_of(Class) }
    it { should_not be_an_instance_of(dummy_class) }
    it { should be_an_instance_of(Object) }
    it { should_not be_a(YourCoolModule) }
  end
#end.call
end

#In this simple case there's necessarily no need for a variable at all..
describe Class.new { include YourCoolModule } do
  it { should respond_to(:your_cool_module_method) }
  it { should_not be_a(Class) }
  it { should be_a(YourCoolModule) }
end

describe "dummy_class not defined" do
  it { defined?(dummy_class).should be_nil }
end

Answer 11

You can also use the helper type

# api_helper.rb
module Api
  def my_meth
    10
  end
end

# spec/api_spec.rb
require "api_helper"

RSpec.describe Api, :type => :helper do
  describe "#my_meth" do
    it { expect( helper.my_meth ).to eq 10 }
  end
end

Here's the documentation: https://www.relishapp.com/rspec/rspec-rails/v/3-3/docs/helper-specs/helper-spec

Answer 12

One possible solution for testing module method which are independent on class that will include them

module moduleToTest
  def method_to_test
    'value'
  end
end

And spec for it

describe moduleToTest do
  let(:dummy_class) { Class.new { include moduleToTest } }
  let(:subject) { dummy_class.new }

  describe '#method_to_test' do
    it 'returns value' do
      expect(subject.method_to_test).to eq('value')
    end
  end
end

And if you want to DRY test them, then shared_examples is good approach

Answer 13

you need to simply include your module to your spec file

module Test
  module MyModule
    def test
      'test'
    end
  end
end

in your spec file

RSpec.describe Test::MyModule do
  include Test::MyModule #you can call directly the method *test*
  it 'returns test' do
    expect(test).to eql('test')
  end
end

Answer 14

This is a recurrent pattern since you're going to need to test more than one module. For that reason, this is more than desirable to create a helper for this.

I found this post that explains how to do it but I'm coping here since the site might be taken down at some point.

This is to avoid the object instances do not implement the instance method: :whatever error you get when trying to allow methods on dummy class.

Code:

In spec/support/helpers/dummy_class_helpers.rb

module DummyClassHelpers

  def dummy_class(name, &block)
    let(name.to_s.underscore) do
      klass = Class.new(&block)

      self.class.const_set name.to_s.classify, klass
    end
  end

end

In spec/spec_helper.rb

# skip this if you want to manually require
Dir[File.expand_path("../support/**/*.rb", __FILE__)].each {|f| require f}

RSpec.configure do |config|
  config.extend DummyClassHelpers
end

In your specs:

require 'spec_helper'

RSpec.shared_examples "JsonSerializerConcern" do

  dummy_class(:dummy)

  dummy_class(:dummy_serializer) do
     def self.represent(object)
     end
   end

  describe "#serialize_collection" do
    it "wraps a record in a serializer" do
      expect(dummy_serializer).to receive(:represent).with(an_instance_of(dummy)).exactly(3).times

      subject.serialize_collection [dummy.new, dummy.new, dummy.new]
    end
  end
end