Add Enumerable#index_by

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@4491 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

activesupport/lib/active_support/core_ext/enumerable.rb

@@ -30,4 +30,19 @@ module Enumerable #:nodoc:
   def sum
     inject(0) { |sum, element| sum + yield(element) }
   end
+  
+  # Convert an enumerable to a hash. Examples:
+  # 
+  #   people.index_by(&:login)
+  #     => { "nextangle" => <Person ...>, "chade-" => <Person ...>, ...}
+  #   people.index_by { |person| "#{person.first_name} #{person.last_name}" }
+  #     => { "Chade- Fowlersburg-e" => <Person ...>, "David Heinemeier Hansson" => <Person ...>, ...}
+  # 
+  def index_by
+    inject({}) do |accum, elem|
+      accum[yield(elem)] = elem
+      accum
+    end
+  end
+  
 end

activesupport/test/core_ext/enumerable_test.rb

@@ -1,4 +1,5 @@
 require 'test/unit'
+require File.dirname(__FILE__) + '/../../lib/active_support/core_ext/symbol'
 require File.dirname(__FILE__) + '/../../lib/active_support/core_ext/enumerable'
 
 Payment = Struct.new(:price)
@@ -24,4 +25,13 @@ class EnumerableTests < Test::Unit::TestCase
     assert_equal 30, payments.sum(&:price)
     assert_equal 60, payments.sum { |p| p.price * 2 }
   end
+  
+  def test_index_by
+    payments = [ Payment.new(5), Payment.new(15), Payment.new(10) ]
+    assert_equal(
+      {5 => payments[0], 15 => payments[1], 10 => payments[2]},
+      payments.index_by(&:price)
+    )
+  end
+  
 end