After searching the interwebs, I found a solution to use
sort_by when you need to sort by multiple field.
I have a model Person with 2 fields: first_name and last_name.
I want to sort an array filled with Person models by last_name and than by first_name.
Well, it's not that hard. Just return an array in the
sort_by block with the fields you want to sort in the order you want to sort them.
people.sort_by do |person| [person.last_name, person.first_name] end