gistfile1.rb

SPEC

  describe "when a membeship is created" do
  
    let!(:another_membership) { FactoryGirl.create(:membership, user: user, membership_type: membertype,  
                                                 start: Date.parse("2013-11-15")) }
  
    context "if there are other memberships associated with user" do
      it "sets the end date for any of them that have a nil end date" do
        expect(Membership.find_by(start: Date.parse("2012-11-15"))).not_to be_nil        
        expect(Membership.find_by(start: Date.parse("2012-11-15")).end).not_to be_nil               
      end
    end
  end

MODEL
  after_create :enforce_at_most_one_open_for_user

  def enforce_at_most_one_open_for_user
    new_end_date = self.start.prev_day
    if !self.user.nil?
      self.user.memberships.each do |m|
        m.end = new_end_date if m.id != self.id && m.nil?
      end
    end
  end