Add each_with_object from 1.9 for a more convenient alternative to inject.

Signed-off-by: Michael Koziarski <michael@koziarski.com>
[#962 state:committed]

activesupport/lib/active_support/core_ext/enumerable.rb

@@ -64,8 +64,28 @@ module Enumerable
     end
   end
 
+  # Iterates over a collection, passing the current element *and* the
+  # +memo+ to the block. Handy for building up hashes or
+  # reducing collections down to one object. Examples:
+  #
+  #   %w(foo bar).each_with_object({}) { |str, hsh| hsh[str] = str.upcase } #=> {'foo' => 'FOO', 'bar' => 'BAR'}
+  #
+  # *Note* that you can't use immutable objects like numbers, true or false as
+  # the memo. You would think the following returns 120, but since the memo is
+  # never changed, it does not.
+  #
+  #   (1..5).each_with_object(1) { |value, memo| memo *= value } # => 1
+  #
+  def each_with_object(memo, &block)
+    returning memo do |memo|
+      each do |element|
+        block.call(element, memo)
+      end
+    end
+  end unless [].respond_to?(:each_with_object)
+
   # 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}" }

activesupport/test/core_ext/enumerable_test.rb

@@ -58,6 +58,11 @@ class EnumerableTests < Test::Unit::TestCase
     assert_equal Payment.new(0), [].sum(Payment.new(0))
   end
 
+  def test_each_with_object
+    result = %w(foo bar).each_with_object({}) { |str, hsh| hsh[str] = str.upcase }
+    assert_equal({'foo' => 'FOO', 'bar' => 'BAR'}, result)
+  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] },