thing_spec_revisited.rb

subject(:valid_things_ids) { described_class.valid_things_ids(group) }

let(:group) { 'blurb' }

let!(:random_thing) { FactoryGirl.create(:something, group: 'blurb', id: 111) }
let!(:another_thing) { FactoryGirl.create(:something, group: 'blurb', id: 222) }
let!(:not_included) { FactoryGirl.create(:something, group: 'shrug', id: 333) }

it do
  expect(valid_things_ids).to include 111
  expect(valid_things_ids).to include 222
  expect(valid_things_ids).not_to include 333
end