Mocking in RSpec.

claim_route_controller_spec.rb

context "on ClaimRoute failure" do
  let(:service_double) { instance_double(ClaimRoute, call: service_result, errors: service_expected_errors) }
  let(:service_result) { false }
  let(:service_expected_errors) { [ClaimRoute::REQUIRED_PARAMETERS_NOT_PROVIDED] }

  before do
    post_claim_route
  end

  it "redirects to the game page" do
    expect(response).to redirect_to game
  end

  it "has the expected errors in the flash with error prefix" do
    expect(flash[:errors]).to eq service_expected_errors_prefixed
  end
end

Snippets taken from: https://github.com/jordancrawfordnz/ticket-to-ride/blob/master/spec/controllers/claim_route_controller_spec.rb