Uniq by object attribute in Ruby

Question

What\'s the most elegant way to select out objects in an array that are unique with respect to one or more attributes?

These objects are stored in ActiveRecord so us

Answer 1

Add the uniq_by method to Array in your project. It works by analogy with sort_by. So uniq_by is to uniq as sort_by is to sort. Usage:

uniq_array = my_array.uniq_by {|obj| obj.id}

The implementation:

class Array
  def uniq_by(&blk)
    transforms = []
    self.select do |el|
      should_keep = !transforms.include?(t=blk[el])
      transforms << t
      should_keep
    end
  end
end

Note that it returns a new array rather than modifying your current one in place. We haven't written a uniq_by! method but it should be easy enough if you wanted to.

EDIT: Tribalvibes points out that that implementation is O(n^2). Better would be something like (untested)...

class Array
  def uniq_by(&blk)
    transforms = {}
    select do |el|
      t = blk[el]
      should_keep = !transforms[t]
      transforms[t] = true
      should_keep
    end
  end
end