Undefined method sort_by in rails

I am facing issues while using sort or sort_by in my presenter and controller. It says undefined method 'sort_by' .

Does it belong to any particular class? I have looked it up online but I am not able to find anything concrete.

Can any one shed light on this?

Here is my presenter code-

def sortDocument()
    @list.sort do |a, b|
      (b.published_date <=> a.published_date) ||
      a.display_name <=> b.display_name
      @list
end

and alternatively

def sortDocument()
  @list.sort_by!{ |m| m.published_date }
end