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

Rails also has a #uniq_by method.

Reference: Parameterized Array#uniq (i.e., uniq_by)

Answer 2

Use Array#uniq with a block:

objects.uniq {|obj| obj.attribute}

Or a more concise approach:

objects.uniq(&:attribute)

Answer 3

Do it on the database level:

YourModel.find(:all, :group => "status")

Answer 4

Now if you can sort on the attribute values this can be done:

class A
  attr_accessor :val
  def initialize(v); self.val = v; end
end

objs = [1,2,6,3,7,7,8,2,8].map{|i| A.new(i)}

objs.sort_by{|a| a.val}.inject([]) do |uniqs, a|
  uniqs << a if uniqs.empty? || a.val != uniqs.last.val
  uniqs
end

That's for a 1-attribute unique, but the same thing can be done w/ lexicographical sort ...

Answer 5

I like jmah and Head's answers. But do they preserve array order? They might in later versions of ruby since there have been some hash insertion-order-preserving requirements written into the language specification, but here's a similar solution that I like to use that preserves order regardless.

h = Set.new
objs.select{|el| h.add?(el.attr)}

Answer 6

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