Is there an expression or idiom for someone who knows everything about one thing?

There's the old joke about the man who learned more and more about less and less until, finally, he knew everything about nothing. He befriended a man who understood less an less about more and more until he knew nothing about everything. Opposites attracted.

A man who knows everything about one thing may be seen as a specialist savant.

Expert

OOOOOOH YOU ARE AWESOME....Of course not expert I said idiom or expression.

All knower.

http://www.thefreedictionary.com/all-knowing

http://www.vocabulary.com/dictionary/all-knowing