Fix bug where reset_counters resets the wrong counter cache.

If a model belongs_to two associations with the same class, then reset_counters
will reset the wrong counter cache.

Finding the right reflection should use the foreign_key instead, which should
be unique.

activerecord/lib/active_record/counter_cache.rb

@@ -25,9 +25,10 @@ module ActiveRecord
           self.name
         end
 
+        foreign_key  = has_many_association.foreign_key.to_s
         child_class  = has_many_association.klass
         belongs_to   = child_class.reflect_on_all_associations(:belongs_to)
-        reflection   = belongs_to.find { |e| e.class_name == expected_name }
+        reflection   = belongs_to.find { |e| e.foreign_key.to_s == foreign_key }
         counter_name = reflection.counter_cache_column
 
         stmt = unscoped.where(arel_table[primary_key].eq(object.id)).arel.compile_update({

activerecord/test/cases/counter_cache_test.rb

@@ -6,9 +6,11 @@ require 'models/engine'
 require 'models/reply'
 require 'models/category'
 require 'models/categorization'
+require 'models/dog'
+require 'models/dog_lover'
 
 class CounterCacheTest < ActiveRecord::TestCase
-  fixtures :topics, :categories, :categorizations, :cars
+  fixtures :topics, :categories, :categorizations, :cars, :dogs, :dog_lovers
 
   class ::SpecialTopic < ::Topic
     has_many :special_replies, :foreign_key => 'parent_id'
@@ -61,7 +63,7 @@ class CounterCacheTest < ActiveRecord::TestCase
     end
   end
 
-  test "reset counter should with belongs_to which has class_name" do
+  test "reset counter with belongs_to which has class_name" do
     car = cars(:honda)
     assert_nothing_raised do
       Car.reset_counters(car.id, :engines)
@@ -71,6 +73,20 @@ class CounterCacheTest < ActiveRecord::TestCase
     end
   end
 
+  test "reset the right counter if two have the same class_name" do
+    david = dog_lovers(:david)
+
+    DogLover.increment_counter(:bred_dogs_count, david.id)
+    DogLover.increment_counter(:trained_dogs_count, david.id)
+
+    assert_difference 'david.reload.bred_dogs_count', -1 do
+      DogLover.reset_counters(david.id, :bred_dogs)
+    end
+    assert_difference 'david.reload.trained_dogs_count', -1 do
+      DogLover.reset_counters(david.id, :trained_dogs)
+    end
+  end
+
   test "update counter with initial null value" do
     category = categories(:general)
     assert_equal 2, category.categorizations.count

activerecord/test/fixtures/dog_lovers.yml

@@ -0,0 +1,4 @@
+david:
+  id: 1
+  bred_dogs_count: 0
+  trained_dogs_count: 1

activerecord/test/fixtures/dogs.yml

@@ -0,0 +1,3 @@
+sophie:
+  id: 1
+  trainer_id: 1

activerecord/test/models/dog.rb

@@ -0,0 +1,4 @@
+class Dog < ActiveRecord::Base
+  belongs_to :breeder, :class_name => "DogLover", :counter_cache => :bred_dogs_count
+  belongs_to :trainer, :class_name => "DogLover", :counter_cache => :trained_dogs_count
+end

activerecord/test/models/dog_lover.rb

@@ -0,0 +1,4 @@
+class DogLover < ActiveRecord::Base
+  has_many :trained_dogs, :class_name => "Dog", :foreign_key => :trainer_id
+  has_many :bred_dogs, :class_name => "Dog", :foreign_key => :breeder_id
+end

activerecord/test/schema/schema.rb

@@ -213,6 +213,16 @@ ActiveRecord::Schema.define do
     t.integer :access_level, :default => 1
   end
 
+  create_table :dog_lovers, :force => true do |t|
+    t.integer :trained_dogs_count, :default => 0
+    t.integer :bred_dogs_count, :default => 0
+  end
+
+  create_table :dogs, :force => true do |t|
+    t.integer :trainer_id
+    t.integer :breeder_id
+  end
+
   create_table :edges, :force => true, :id => false do |t|
     t.column :source_id, :integer, :null => false
     t.column :sink_id,   :integer, :null => false