A or B, with reasons

a.rb

# A. Less nested contexts, clearer object collaboration at the cost of repetition

describe PlaceParameters do

  describe '#sort_by' do

    it 'concatenates the sort_by and sort_dir params' do
      params = { sort_by: 'name', sort_dir: 'asc' }
      sort_by = PlaceParameters.new(params).sort_by
      expect(sort_by).to match /^name asc$/i
    end

    it 'explicitly defaults sort direction to ASC' do
      params = { sort_by: 'name' }
      sort_by = PlaceParameters.new(params).sort_by
      expect(sort_by).to match /^name asc$/i
    end

  end

end

b.rb

# B. Contexts used to DRY assertions, object collaboration less obvious at a glance

describe PlaceParameters do

  let(:place_parameters) { PlaceParameters.new(params) }

  describe '#sort_by' do

    context 'when sort_by and sort_dir are specified' do
      let(:params) { {sort_by: 'name', sort_dir: 'asc'} }

      it 'concatenates them' do
        expect(place_parameters.sort_by).to match /^name asc$/i
      end
    end

    context 'when sort_dir is not specified' do
      let(:params) { {sort_by: 'name'} }

      it 'explicitly defaults sort direction to ASC' do
        expect(place_parameters.sort_by).to match /^name asc$/i
      end
    end

  end

end

I prefer B for the following reasons:

• DRY
• it block is lean and very easy to understand as it's an assertion only (no setup)
• Use of let's which is generally encouraged (http://betterspecs.org/)

To play devil's advocate:

• A makes the collaboration of params and PlaceParams much more obvious, more accurately reflecting how you'd use the two objects in real implementation code.
• A is, with the same attention to clarity in both the test layout and the resulting output, far less lines (30%)

B ditto @ashmckenzie. The expectation is much clearer

A. I value tests that are easy to read and change over DRY.

With B, I have to jump to two different lines outside of the test to work out what is going on.

cheers for having a look, @compactcode. Although I'm on the fence, I share that exact problem

@gabrielrotbart suggested this.

It still uses the contexts to set up each example differently, but repeats the interaction between the PlaceParameter and the params object for clarity - mirroring implementation code:

describe PlaceParameters do

  describe '#sort_by' do

    context 'when sort_by and sort_dir are specified' do
      let(:params) { {sort_by: 'name', sort_dir: 'asc'} }

      it 'concatenates them' do
        sort_by = PlaceParameters.new(params).sort_by
        expect(sort_by).to match /^name asc$/i
      end
    end

    context 'when sort_dir is not specified' do
      let(:params) { {sort_by: 'name'} }

      it 'explicitly defaults sort direction to ASC' do
        sort_by = PlaceParameters.new(params).sort_by
        expect(sort_by).to match /^name asc$/i
      end
    end

  end

end