factories.rb

  factory :user do
    name "Jessie Pinkman"
    email
    password "secret"
    password_confirmation "secret"

    factory :group_admin, parent: :user do
      ignore do
        group = nil
      end

      after(:create) { |user, evaluator| user.add_role :admin, evaluator.group }
    end
  end

group_policy_spec.rb

require 'spec_helper'

describe GroupPolicy do
  subject { GroupPolicy.new(user, group) }

  let(:group) { FactoryGirl.create :group }

  context 'A visitor' do
    let(:user) { FactoryGirl.create :user }

    it { should     permit :create }
    it { should_not permit :show }
    it { should_not permit :update }
    it { should_not permit :destroy }
  end

  context 'A group admin' do
    let(:user) do
      FactoryGirl.create :group_admin, group: group
    end

    it { should permit :create }
    it { should permit :show }
    it { should permit :update }
    it { should permit :destroy }
  end
end

  3) GroupPolicy A group admin 
     Failure/Error: FactoryGirl.create :group_admin, group: group
     NoMethodError:
       undefined method `group=' for #<User:0x0000000498b4e0>
     # ./spec/policies/group_policy_spec.rb:19:in `block (3 levels) in <top (required)>'
     # ./spec/policies/group_policy_spec.rb:4:in `block (2 levels) in <top (required)>'
     # ./spec/policies/group_policy_spec.rb:25:in `block (3 levels) in <top (required)>'