Actually two...he did Princess Bride also.
There is a five-hour workprint for Spinal Tap.
Rob was lucky enough to stand aside and let the comedy troupe do their thing. The writing credits as assigned to Christopher Guest, Michael McKean, Harry Shearer, & Rob Reiner. He had more than enough material to work with in editing it down to a 90 minute film (although some elements of edited plot lines remain).
Three of those guys have gone on to make similar films. One of them has not shown the same briliance.