shared_context 'model contract' do
  shared_examples_for 'is not writable' do
    before do
      instance.model.attributes = { attribute => value }
    end

    it 'explains the not writable error' do
      instance.validate
      expect(instance.errors.details[attribute])
        .to match_array [{ error: :error_readonly }]
    end
  end

  shared_examples_for 'is writable' do
    before do
      instance.model.attributes = { attribute => value }
    end

    it 'is writable' do
      instance.validate

      expect(instance.errors.details[attribute])
        .not_to include(error: :error_readonly)
    end
  end
end