use columns hash to look up the column for the count field

activerecord/lib/active_record/relation/calculations.rb

@@ -343,7 +343,7 @@ def column_alias_for(*keys)
 
     def column_for(field)
       field_name = field.respond_to?(:name) ? field.name.to_s : field.to_s.split('.').last
-      @klass.columns.detect { |c| c.name.to_s == field_name }
+      @klass.columns_hash[field_name]
     end
 
     def type_cast_calculated_value(value, column, operation = nil)

activerecord/test/cases/calculations_test.rb

@@ -6,6 +6,8 @@
 require 'models/organization'
 require 'models/possession'
 require 'models/topic'
+require 'models/minivan'
+require 'models/speedometer'
 
 Company.has_many :accounts
 
@@ -239,21 +241,12 @@ def test_should_calculate_grouped_association_with_invalid_field
   end
 
   def test_should_group_by_association_with_non_numeric_foreign_key
-    ActiveRecord::Base.connection.expects(:select_all).returns([{"count_all" => 1, "firm_id" => "ABC"}])
+    firm = Speedometer.create! id: 'ABC'
+    mv  = Minivan.create! id: 'OMG', speedometer_id: 'ABC'
 
-    firm = mock()
-    firm.expects(:id).returns("ABC")
-    firm.expects(:class).returns(Firm)
-    Company.expects(:find).with(["ABC"]).returns([firm])
-
-    column = mock()
-    column.expects(:name).at_least_once.returns(:firm_id)
-    column.expects(:type_cast).with("ABC").returns("ABC")
-    Account.expects(:columns).at_least_once.returns([column])
-
-    c = Account.group(:firm).count(:all)
+    c = Minivan.group(:speedometer).count(:all)
     first_key = c.keys.first
-    assert_equal Firm, first_key.class
+    assert_equal Speedometer, first_key.class
     assert_equal 1, c[first_key]
   end