Calculations: return nil average instead of 0 when there are no rows to average. Closes #8298.

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@6904 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

activerecord/CHANGELOG

 *SVN*
 
+* Calculations: return nil average instead of 0 when there are no rows to average.  #8298 [davidw]
+
 * acts_as_nested_set: direct_children is sorted correctly.  #4761 [Josh Peek, rails@33lc0.net]
 
 * Raise an exception if both attr_protected and attr_accessible are declared.  #8507 [stellsmi]

activerecord/lib/active_record/calculations.rb

@@ -265,7 +265,7 @@ def type_cast_calculated_value(value, column, operation = nil)
           operation = operation.to_s.downcase
           case operation
             when 'count' then value.to_i
-            when 'avg'   then value.to_f
+            when 'avg'   then value && value.to_f
             else column ? column.type_cast(value) : value
           end
         end

activerecord/test/calculations_test.rb

@@ -4,6 +4,10 @@
 
 Company.has_many :accounts
 
+class NumericData < ActiveRecord::Base
+  self.table_name = 'numeric_data'
+end
+
 class CalculationsTest < Test::Unit::TestCase
   fixtures :companies, :accounts, :topics
 
@@ -17,6 +21,10 @@ def test_should_average_field
     assert_in_delta 53.0, value, 0.001
   end
 
+  def test_should_return_nil_as_average
+    assert_nil NumericData.average(:bank_balance)
+  end
+
   def test_should_get_maximum_of_field
     assert_equal 60, Account.maximum(:credit_limit)
   end