He was seeing if you had a depth of understanding. I do that too.
I want to see that people are aware of what's going on below them. Not that it's just "magic" to some keyboard basher. ruby on rails attracts an extreme number of keyboard bashers, so they get extra scrutiny.
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.
I get that, but when the interviewee is not just a "keyboard basher" then it suggests that the interviewer is the one lacking depth of understanding as they are conflating superficial ignorance with fundamental incompetence.
I want to see that people are aware of what's going on below them. Not that it's just "magic" to some keyboard basher. ruby on rails attracts an extreme number of keyboard bashers, so they get extra scrutiny.