Fix counter_cache double increment bug

activerecord/lib/active_record/associations/belongs_to_association.rb

@@ -61,6 +61,7 @@ def require_counter_update?
 
         def update_counters_on_replace(record)
           if require_counter_update? && different_target?(record)
+            owner.instance_variable_set :@_after_replace_counter_called, true
             record.increment!(reflection.counter_cache_column)
             decrement_counters
           end

activerecord/lib/active_record/associations/builder/belongs_to.rb

@@ -33,6 +33,8 @@ def belongs_to_counter_cache_after_update(reflection)
 
           if (@_after_create_counter_called ||= false)
             @_after_create_counter_called = false
+          elsif (@_after_replace_counter_called ||= false)
+            @_after_replace_counter_called = false
           elsif attribute_changed?(foreign_key) && !new_record?
             if reflection.polymorphic?
               model     = attribute(reflection.foreign_type).try(:constantize)

activerecord/test/cases/associations/belongs_to_associations_test.rb

@@ -700,6 +700,17 @@ def test_custom_counter_cache
     assert_equal 17, reply.replies.size
   end
 
+  def test_replace_counter_cache
+    topic = Topic.create(title: "Zoom-zoom-zoom")
+    reply = Reply.create(title: "re: zoom", content: "speedy quick!")
+
+    reply.topic = topic
+    reply.save
+    topic.reload
+
+    assert_equal 1, topic.replies_count
+  end
+
   def test_association_assignment_sticks
     post = Post.first