1 files changed, 14 insertions, 7 deletions
diff --git a/activesupport/lib/active_support/core_ext/enumerable.rb b/activesupport/lib/active_support/core_ext/enumerable.rb
index c3a351538f..35a8e0538c 100644
--- a/activesupport/lib/active_support/core_ext/enumerable.rb
+++ b/activesupport/lib/active_support/core_ext/enumerable.rb
@@ -15,15 +15,23 @@ module Enumerable
   #   "2006-02-24 -> Transcript, Transcript"
   #   "2006-02-23 -> Transcript"
   def group_by
-    inject([]) do |groups, element|
-      value = yield(element)
-      if (last_group = groups.last) && last_group.first == value
-        last_group.last << element
+    groups = []
+
+    inject({}) do |grouped, element|
+      index = yield(element)
+
+      if group = grouped[index]
+        group << element
       else
-        groups << [value, [element]]
+        group = [element]
+        groups << [index, group]
+        grouped[index] = group
       end
-      groups
+
+      grouped
     end
+
+    groups
   end if RUBY_VERSION < '1.9'
 
   # Calculates a sum from the elements. Examples:
@@ -64,5 +72,4 @@ module Enumerable
       accum
     end
   end
-  
 end