Factory.sequence(:email) {|n| "person#{n}@example.com" }

Factory.define :role do |r|
  r.name {"Admin"}
end

Factory.define :role_user do |ru|
  ru.association(:role)
  ru.association(:user)
end

Factory.define :user do |u|
  u.first_name {"matt"}
  u.last_name {"jones"}
  u.email {Factory.next(:email)}
end
and using it like

  let(:role_user) { FactoryGirl.create(:role_user)  }
  before { sign_in(role_user) }