Sorting an array of structs

Question

I have an array of structs called leaders. The struct class looks like this, for contextual info:

class Leader < Struct.new(:rank, :user); end

Two questions:

1. How do I sort the array of structs by rank?
2. How do I sort the array of structs by rank and by user.created_at?

Answer 1

1.

Assuming rank is numeric:

array.sort {| a, b | a[:rank] <=> b[:rank] }

This is basically just specifying that we compare a and b using [:rank].

2. 

array.sort {| a, b | a[:rank] == b[:rank] ? 
    a[:user].created_at <=> b[:user].created_at : 
    a[:rank} <=> b[:rank] }

This uses a ternary. If the ranks are equal, we compare by [:user].created_at. Otherwise, we compare by the ranks.

You could implement <=> in your own class to allow sorting natively:

class Leader < Struct.new(:rank, :user)
  def <=>(other)
    self[:rank] <=> other[:rank]
  end
end

Then you can do:

leaders.sort()

If you include Comparable, it will provide the other comparison operators too.

Answer 2

Here's an implementation using sort_by. As noted in the documentation, it may be less efficient than a version using sort, depending on how expensive it is to retrieve the rank and created_at values.

Sorting by rank:

leaders.sort_by {|l| l.rank}

Sorting by rank and created_at:

leaders.sort_by {|l| [l.rank, l.user.created_at]}