Como testar adequadamente as habilidades do CanCan com o RSpec

Estou testando as habilidades do CanCan pela primeira vez e estou perplexo. Estou sentindo falta de alguma coisa ... mesmo que eu retorne false / true dentro da lata: invite_to block Ainda não estou recebendo especificações passadas. Estou faltando usar os fósforos CanCan? ou tocos? ou definiing habilidades no CanCan?

Algo que estou sentindo falta?

ability.rb

class Ability
  include CanCan::Ability

  def initialize(user)
    user ||= User.new

    can :invite_to, Network do |network|
      network.allows_invitations? && (user.admin? || user.can_send_invitations_for?(network))
    end
  end
end

ability_spec.rb

require 'cancan'
require 'cancan/matchers'
require_relative '../../app/models/ability.rb'

class Network; end;

describe Ability do
  let(:ability) { Ability.new(@user) }

  describe "#invite_to, Network" do
    context "when network level invitations are enabled" do
      let(:network) { stub(allows_invitations?: true) }

      it "allows an admin" do
        @user = stub(admin?: true)
        ability.should be_able_to(:invite_to, network)
      end

      it "allows a member if the member's invitation privileges are enabled" do
        @user = stub(admin?: false, can_send_invitations_for?: true)
        ability.should be_able_to(:invite_to, network)
      end

      it "denies a member if the member's invitation privileges are disabled" do
        @user = stub(admin?: false, can_send_invitations_for?: false)
        ability.should_not be_able_to(:invite_to, network)
      end
    end
  end
end

Falhas

  1) Ability#invite_to, Network when network level invitations are enabled allows an admin
     Failure/Error: ability.should be_able_to(:invite_to, network)
       expected to be able to :invite_to #<RSpec::Mocks::Mock:0x3fe3ed90444c @name=nil>
     # ./spec/models/ability_spec.rb:16:in `block (4 levels) in <top (required)>'

  2) Ability#invite_to, Network when network level invitations are enabled allows a member if the member's invitation privileges are enabled
     Failure/Error: ability.should be_able_to(:invite_to, network)
       expected to be able to :invite_to #<RSpec::Mocks::Mock:0x3fe3edc27408 @name=nil>
     # ./spec/models/ability_spec.rb:21:in `block (4 levels) in <top (required)>'

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