spec.rb

require 'spec_helper'

describe Purchase do

  before :all do
    @city = Factory.create(:city)
    @user = Factory.create(:user, :cities => [@city])
    @ticket = Factory.create(:ticket)
  end

  let!(:city)   { @city }
  let!(:user)   { @user }
  let!(:ticket) { @ticket }

  subject { Factory.create(:purchase, :user => user, :ticket => ticket, :city => city) }

  # some tests here
  ... 

  describe "nested context" do

    let!(:user) { raise "foo".inspect }

    it "should do stuff" do
      # subject.user will still be the original old one here :(
    end

  end

end