One of the most handy websites out there for people trying to find specific API’s in Ruby is ruby-doc.org.
From the main page, you can search or select your Ruby version.
From the core page, you can narrow down the class or method you’re looking for.
From the std-lib page, you can read docs and see where different aspects of the programming language are implemented.
On each class’ page, you can find the specific functionality you need in the menu on the left.