How to determine if one array contains all elements of another array

Given: a1 = [5, 1, 6, 14, 2, 8] I would like to determine if it contains all elements of: a2 = [2, 6, 15] In this case the result is false. Are there any built-in Ruby/Rails methods to identify such array inclusion? One way to implement this is: a2.index{ |x| !a1.include?(x) }.nil? Is there a better, more readable, way?