instance_variable_set is your friend

If you’ve got some code like this that sets an instance variable, it can be a problem to test.

def self.amqp
  @amqp ||= open_connection
end

If you mock open_connection, you would hope that it would avoid making the call, but if that variable is set, it’s going to ignore the mock. This is more of a problem when running the tests on the command line (instead of by itself in Textmate).

  describe "amqp" do
    it "should create a new amqp" do
      a = mock("amqp")
      Messaging.should_receive(:open_connection).and_return(a)
      Messaging.amqp.should == a
    end
  end

I had another test that was setting @amqp and it would cause my current test to fail. I needed to find a way to make sure that @amqp was nil every time the test was run.

describe "amqp" do
    it "should create a new amqp" do
## reset the @amqp variable to make sure we have it in a known state
      Messaging.instance_variable_set(:@amqp, nil)
##
      a = mock("amqp")
      Messaging.should_receive(:open_connection).and_return(a)
      Messaging.amqp.should == a
    end
  end

instance_variable_set will let you reset that variable without trying to mess around, which I had been doing for a while. This works really well when you’re trying to test cached things as well.