Sorting By Multiple Conditions in Ruby

Question

I have a collection of Post objects and I want to be able to sort them based on these conditions:

• First, by category (news, events, labs, portfolio, etc.)
• Then by date, if date, or by position, if a specific index was set for it

Some posts will have dates (news and events), others will have explicit positions (labs, and portfolio).

I want to be able to call posts.sort!, so I've overridden <=>, but am looking for the most effective way of sorting by these conditions. Below is a pseudo method:

def <=>(other)
  # first, everything is sorted into 
  # smaller chunks by category
  self.category <=> other.category

  # then, per category, by date or position
  if self.date and other.date
    self.date <=> other.date
  else
    self.position <=> other.position
  end
end

It seems like I'd have to actually sort two separate times, rather than cramming everything into that one method. Something like sort_by_category, then sort!. What is the most ruby way to do this?

Answer 1

You should always sort by the same criteria to insure a meaningful order. If comparing two nil dates, it is fine that the position will judge of the order, but if comparing one nil date with a set date, you have to decide which goes first, irrespective of the position (for example by mapping nil to a day way in the past).

Otherwise imagine the following:

a.date = nil                   ; a.position = 1
b.date = Time.now - 1.day      ; b.position = 2
c.date = Time.now              ; c.position = 0

By your original criteria, you would have: a < b < c < a. So, which one is the smallest??

You also want to do the sort at once. For your <=> implementation, use #nonzero?:

def <=>(other)
  return nil unless other.is_a?(Post)
  (self.category <=> other.category).nonzero? ||
  ((self.date || AGES_AGO) <=> (other.date || AGES_AGO)).nonzero? ||
  (self.position <=> other.position).nonzero? ||
  0
end

If you use your comparison criteria just once, or if that criteria is not universal and thus don't want to define <=>, you could use sort with a block:

post_ary.sort{|a, b| (a.category <=> ...).non_zero? || ... }

Better still, there is sort_by and sort_by! which you can use to build an array for what to compare in which priority:

post_ary.sort_by{|a| [a.category, a.date || AGES_AGO, a.position] }

Besides being shorter, using sort_by has the advantage that you can only obtain a well ordered criteria.

Notes:

• sort_by! was introduced in Ruby 1.9.2. You can require 'backports/1.9.2/array/sort_by' to use it with older Rubies.
• I'm assuming that Post is not a subclass of ActiveRecord::Base (in which case you'd want the sort to be done by the db server).