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

If I understand your question correctly, I've tackled this problem using the quasi-hacky approach of comparing the Marshaled objects to determine if any attributes vary. The inject at the end of the following code would be an example:

class Foo
  attr_accessor :foo, :bar, :baz

  def initialize(foo,bar,baz)
    @foo = foo
    @bar = bar
    @baz = baz
  end
end

objs = [Foo.new(1,2,3),Foo.new(1,2,3),Foo.new(2,3,4)]

# find objects that are uniq with respect to attributes
objs.inject([]) do |uniqs,obj|
  if uniqs.all? { |e| Marshal.dump(e) != Marshal.dump(obj) }
    uniqs << obj
  end
  uniqs
end

Answer 2

You can use a hash, which contains only one value for each key:

Hash[*recs.map{|ar| [ar[attr],ar]}.flatten].values

Answer 3

Use Array#uniq with a block:

@photos = @photos.uniq { |p| p.album_id }

Answer 4

The most elegant way I have found is a spin-off using Array#uniq with a block

enumerable_collection.uniq(&:property)

…it reads better too!

Answer 5

ActiveSupport implementation:

def uniq_by
  hash, array = {}, []
  each { |i| hash[yield(i)] ||= (array << i) }
  array
end

Answer 6

I had originally suggested using the select method on Array. To wit:

[1, 2, 3, 4, 5, 6, 7].select{|e| e%2 == 0} gives us [2,4,6] back.

But if you want the first such object, use detect.

[1, 2, 3, 4, 5, 6, 7].detect{|e| e>3} gives us 4.

I'm not sure what you're going for here, though.