Rspec: Shared Examples and Contexts

The best part of choosing Rspec as your tool for automated testing, is the possibility to describe your expectations with very readable sentences. Today, We will present to you two components of Rspec that can make it even more doable.

Shared Contexts

Shared contexts is a way to stick to the don’t repeat yourself rule. It allows us to reuse some test boilerplate such as let, subject, described_class, and other generalisms, for different sets of tests. Let’s see the example below:

# app/models/bottle.rb

class Bottle < ApplicationRecord 

MAX_CAPACITY = 330

  def full?

   current_content >= MAX_CAPACITY

  end

end

# app/models/bowl.rb

class Bowl < ApplicationRecord

  MAX_CAPACITY = 600

  def full?

    current_content >= MAX_CAPACITY

  end

End

Let’s imagine two models, Bottle and Bowl. As containers, they share a common method called full?. Let’s see how we could write our tests using shared_context:

 
# spec/support/shared_contexts/filled.rb

 RSpec.shared_context 'filled' do

   let(:current_capacity) { 50 }

   subject { described_class.new(current_capacity: current_capacity) }

 end

 # spec/models/bottle_spec.rb

 RSpec.describe Bottle, type: :model do

  describe 'full?' do

    context 'when the bottle is' do

      context 'not full' do

       include_context 'filled'

       it 'returns false' do

          expect(subject).to be_falsey

       end

    end

      context 'is full' do

       include_context 'filled' do

        let(:current_capacity) { 330 }

       end

       it 'returns false' do

        expect(subject).to be_falsey

      end

    end

    end

   end

 end

# spec/models/bowl_spec.rb

RSpec.describe Bowl, type: :model do

  describe 'filled?' do

   context 'when the bowl is' do

    context 'not full' do

     include_context 'filled'

      it 'returns false' do

       expect(subject).to be_falsey

     end

   end

   context 'is full' do

     include_context 'filled' do

      let(:current_capacity) { 600 }

<pre><code class="language-ruby">   end</code></pre>

     it 'returns true' do

       expect(subject).to be_truthy

     end

    end

  end

 end

End

See? The tests are using the same configuration context and build different objects to run correctly. Easy and clean, no? Perfect but, reading the specs you could say: Ok, but the tests per si seem pretty equal to me and duplicated no? I need to agree with you. The next topic will help us to improve that.

Shared Examples

Shared examples allow us to reuse a bunch of tests for multiple features of our applications. Using the same example above, we could have something like this:

# spec/support/shared_examples/fillable_spec.rb

RSpec.shared_examples 'Fillable' do |received_capacity|

  describe 'full?' do

    context "when the #{described_class.downcase}" is do

      context 'not full' do

        include_context 'filled'

        it 'returns false' do

          expect(subject).to be_falsey

        end

      end
  

      context 'is full' do

        include_context 'filled' do

          let(:current_capacity) { received_capacity }

        end

        it 'returns true' do

          expect(subject).to be_truthy

        end

      end

    end

  end

end

# spec/models/bottle_spec.rb

RSpec.describe Bottle, type: :model do

  it_behaves_like 'Fillable', 330

end

# spec/models/bowl_spec.rb

RSpec.describe Bowl, type: :model do

  it_behaves_like 'Fillable', 600

end

Clean and tidy code so when you look at it a year from now you’ll be proud. Shared context and examples are great tools to DRY up your code, make it more readable and pleasure to work with it. As with any examples, it’s best practice to be cautious about the complexity of the abstractions when using these tools. If you need to override variables of the context all the time, or over-configure the call to the shared examples, it could indicate you are in search of a different abstraction solution.

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