That's very different from grilling someone about b-trees, though. B-trees are fairly irrelevant to Rails development; it'd be like grilling the interviewee on assembly.

That's not to say that you don't need to weed out the people who only know how to recite various incantations to make magic happen; a knowledge of what's actually happening is very important. But at this level of abstraction, the correct focus should be on different, more relevant things: HTTP, caching, security, etc, which Rails (I'm guessing) handles for you but you should still know.