Fix for has_many_through counter_cache bug

This commit fixes reported issue rails#7630 in which counter
caches were not being updated properly when replacing
has_many_through relationships

activerecord/CHANGELOG.md

@@ -1,5 +1,10 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   Fix counter cache columns not updated when replacing `has_many :through`
+    associations.
+
+    *Matthew Robertson*
+
 *   Recognize migrations placed in directories containing numbers and 'rb'.
     Fix #8492
 

activerecord/lib/active_record/associations/has_many_through_association.rb

@@ -153,6 +153,11 @@ def delete_records(records, method)
 
           delete_through_records(records)
 
+          if source_reflection.options[:counter_cache]
+            counter = source_reflection.counter_cache_column
+            klass.decrement_counter counter, records.map(&:id)
+          end
+
           if through_reflection.macro == :has_many && update_through_counter?(method)
             update_counter(-count, through_reflection)
           end

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -330,6 +330,17 @@ def test_update_counter_caches_on_delete_with_dependent_nullify
     end
   end
 
+  def test_update_counter_caches_on_replace_association
+    post = posts(:welcome)
+    tag  = post.tags.create!(:name => 'doomed')
+    tag.tagged_posts << posts(:thinking)
+
+    tag.tagged_posts = []
+    post.reload
+
+    assert_equal(post.taggings.count, post.taggings_count)
+  end
+
   def test_replace_association
     assert_queries(4){posts(:welcome);people(:david);people(:michael); posts(:welcome).people(true)}