title: Django prefetch_related and select_related
body: <p>I am wondering about the behaviour of prefetch_related() and select_related(). 
  If I do something like Model.objects.filter(...).prefetch_related(), I notice that there are much less database queries occurring. So my initial guess is that, if one does not specify the needed look-ups in prefetch_related(), it will automatically go through all of the model fields and do the needed job. However, I cannot find any reference to it on the web, which seems pretty strange to me. Is my guess correct or am I missing something? </p>