Ensure Associations#sum returns 0 when no rows are returned. [#295 state:resolved]

Signed-off-by: Pratik Naik <pratiknaik@gmail.com>

activerecord/lib/active_record/calculations.rb

@@ -71,7 +71,7 @@ module ActiveRecord
       #
       #   Person.sum('age')
       def sum(column_name, options = {})
-        calculate(:sum, column_name, options) || 0
+        calculate(:sum, column_name, options)
       end
 
       # This calculates aggregate values in the given column.  Methods for count, sum, average, minimum, and maximum have been added as shortcuts.
@@ -265,8 +265,8 @@ module ActiveRecord
         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 && value.to_f
+            when 'count', 'sum' then value.to_i
+            when 'avg'          then value && value.to_f
             else column ? column.type_cast(value) : value
           end
         end

activerecord/test/cases/calculations_test.rb

@@ -99,6 +99,7 @@ class CalculationsTest < ActiveRecord::TestCase
 
   def test_should_return_zero_if_sum_conditions_return_nothing
     assert_equal 0, Account.sum(:credit_limit, :conditions => '1 = 2')
+    assert_equal 0, companies(:rails_core).companies.sum(:id, :conditions => '1 = 2')
   end
 
   def test_should_group_by_summed_field_with_conditions
@@ -266,6 +267,6 @@ class CalculationsTest < ActiveRecord::TestCase
   end
 
   def test_should_sum_expression
-    assert_equal "636", Account.sum("2 * credit_limit")
+    assert_equal 636, Account.sum("2 * credit_limit")
   end
 end