Added Enumerable#count_by and tests

activesupport/lib/active_support/core_ext/enumerable.rb

@@ -35,6 +35,36 @@ def group_by
     assoc
   end unless [].respond_to?(:group_by)
 
+  # Return the counts of the objects in the enumerable matching the result of 
+  # the block. Useful, for example, for counting the parents by their number of 
+  # children or counting users by their status.
+  #
+  # Examples:
+  #
+  #   parents.count_by do |parent|
+  #     parent.children.count
+  #   end
+  #   "1 -> 12"
+  #   "2 -> 8"
+  #   "3 -> 10"
+  #   "7 -> 1"
+  #
+  #   users.count_by(&:status)
+  #   "active -> 37"
+  #   "inactive -> 4"
+  #   "blacklisted -> 2"
+  #   "pending -> 17"
+  def count_by(&block)
+    counts = {}
+
+    grouped = group_by(&block)
+    grouped.each do |key, vals|
+      counts[key] = vals.size
+    end
+
+    counts
+  end
+
   # Calculates a sum from the elements. Examples:
   #
   #  payments.sum { |p| p.price * p.tax_rate }

activesupport/test/core_ext/enumerable_test.rb

@@ -27,6 +27,26 @@ def test_group_by
     assert({}.merge(grouped), "Could not convert ActiveSupport::OrderedHash into Hash")
   end
 
+  def test_count_by
+    names = %w(alexander andy bob jacob janet joe mel peter susan)
+
+    length_counts = names.count_by(&:length)
+
+    assert_equal [3, 4, 5, 9], length_counts.keys.sort
+    assert_equal length_counts[3], 3
+    assert_equal length_counts[4], 1
+    assert_equal length_counts[5], 4
+    assert_equal length_counts[9], 1
+
+    letter_counts = names.count_by { |name| name.first }
+    assert_equal %w(a b j m p s), letter_counts.keys.sort
+    assert_equal letter_counts["a"], 2
+    assert_equal letter_counts["b"], 1
+    assert_equal letter_counts["j"], 3
+    assert_equal letter_counts["m"], 1
+    assert_equal letter_counts["s"], 1
+  end
+
   def test_sums
     assert_equal 30, [5, 15, 10].sum
     assert_equal 30, [5, 15, 10].sum { |i| i }