Fix `belongs_to_counter_cache_after_update` to respect polymorphic type change

4 files changed, 21 insertions, 3 deletions

diff --git a/activerecord/lib/active_record/associations/belongs_to_association.rb b/activerecord/lib/active_record/associations/belongs_to_association.rb
index 121592733c..cea2570ac4 100644
--- a/activerecord/lib/active_record/associations/belongs_to_association.rb
+++ b/activerecord/lib/active_record/associations/belongs_to_association.rb
@@ -42,6 +42,10 @@ module ActiveRecord
         update_counters(1)
       end
 
+      def target_changed?
+        owner.saved_change_to_attribute?(reflection.foreign_key)
+      end
+
       private
         def replace(record)
           if record
diff --git a/activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb b/activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb
index 75b4c4481a..3fd2fb5f67 100644
--- a/activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb
+++ b/activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb
@@ -9,8 +9,11 @@ module ActiveRecord
         type.presence && type.constantize
       end
 
-      private
+      def target_changed?
+        super || owner.saved_change_to_attribute?(reflection.foreign_type)
+      end
 
+      private
         def replace_keys(record)
           super
           owner[reflection.foreign_type] = record ? record.class.polymorphic_name : nil
diff --git a/activerecord/lib/active_record/associations/builder/belongs_to.rb b/activerecord/lib/active_record/associations/builder/belongs_to.rb
index 1dd6e5bed8..9e012285e5 100644
--- a/activerecord/lib/active_record/associations/builder/belongs_to.rb
+++ b/activerecord/lib/active_record/associations/builder/belongs_to.rb
@@ -36,7 +36,7 @@ module ActiveRecord::Associations::Builder # :nodoc:
 
           if (@_after_replace_counter_called ||= false)
             @_after_replace_counter_called = false
-          elsif saved_change_to_attribute?(foreign_key) && !new_record?
+          elsif association(reflection.name).target_changed?
             if reflection.polymorphic?
               model     = attribute_in_database(reflection.foreign_type).try(:constantize)
               model_was = attribute_before_last_save(reflection.foreign_type).try(:constantize)
diff --git a/activerecord/test/cases/associations/belongs_to_associations_test.rb b/activerecord/test/cases/associations/belongs_to_associations_test.rb
index 856bc70f6b..6cbde2796a 100644
--- a/activerecord/test/cases/associations/belongs_to_associations_test.rb
+++ b/activerecord/test/cases/associations/belongs_to_associations_test.rb
@@ -1071,9 +1071,20 @@ class BelongsToAssociationsTest < ActiveRecord::TestCase
     post    = posts(:welcome)
     comment = comments(:greetings)
 
-    assert_difference lambda { post.reload.tags_count }, -1 do
+    assert_equal post.id, comment.id
+
+    assert_difference "post.reload.tags_count", -1 do
       assert_difference "comment.reload.tags_count", +1 do
         tagging.taggable = comment
+        tagging.save!
+      end
+    end
+
+    assert_difference "comment.reload.tags_count", -1 do
+      assert_difference "post.reload.tags_count", +1 do
+        tagging.taggable_type = post.class.polymorphic_name
+        tagging.taggable_id = post.id
+        tagging.save!
       end
     end
   end