I want to share something I just figured out. I had to write some code that used a class from a plugin if it was there and but did not require the plugin. The problem I faced was how to test this.

My method looked like this

class User 
 def lookup_additional_person_info
   if defined? CommonServices::Person
     person = CommonServices::Person.find_by_id(@person_id)
     @address = person.address
   else
     @address = 'not available'
   end
 end
end

I knew how to define CommonServices::Person to test the positive case but what about the negative? I found out you can undefine a class by sending its parent :remove_constant

  it 'should default additional parameters if CommonServices::Person is not defined' do
   defined?(CommonServices::Person).should be_nil
   user = User.new(:person_id=123)
   user.lookup_additional_person_info
   user.address.should == 'not available'
 end

 it 'should not populate additional parameters if CommonServices::Person is not defined' do
   begin
     defined?(CommonServices::Person).should be_nil
     module CommonServices
       class Person
       end
     end
     CommonServices::Person.expects(:find).with(123).returns(person=mock)
     person.expects(:address).returns(address_from_service=mock)
  
     user = User.new(:person_id=123)
     user.lookup_additional_person_info
     user.address.should == address_from_service
   ensure
     CommonServices.send(:remove_const, :Person)
   end
 end