Ordering.by

gistfile1.scala

scala> case class Foo(n: Int)
defined class Foo

scala> List[Foo]().sorted
<console>:10: error: No implicit Ordering defined for Foo.
              List[Foo]().sorted
                          ^

scala> implicit def ordFoo = Ordering.by[Foo, Int](_.n)
ordFoo: scala.math.Ordering[Foo]

scala> List[Foo]().sorted
res1: List[Foo] = List()