Merge Pull Request #16445

1 files changed, 12 insertions, 4 deletions

diff --git a/activerecord/lib/active_record/associations/builder/belongs_to.rb b/activerecord/lib/active_record/associations/builder/belongs_to.rb
index 97eb007f62..6e4a53f7fb 100644
--- a/activerecord/lib/active_record/associations/builder/belongs_to.rb
+++ b/activerecord/lib/active_record/associations/builder/belongs_to.rb
@@ -33,16 +33,24 @@ module ActiveRecord::Associations::Builder
 
           if (@_after_create_counter_called ||= false)
             @_after_create_counter_called = false
-          elsif attribute_changed?(foreign_key) && !new_record? && reflection.constructable?
-            model           = reflection.klass
+          elsif attribute_changed?(foreign_key) && !new_record?
+            if reflection.polymorphic?
+              model     = attribute(reflection.foreign_type).try(:constantize)
+              model_was = attribute_was(reflection.foreign_type).try(:constantize)
+            else
+              model     = reflection.klass
+              model_was = reflection.klass
+            end
+
             foreign_key_was = attribute_was foreign_key
             foreign_key     = attribute foreign_key
 
             if foreign_key && model.respond_to?(:increment_counter)
               model.increment_counter(cache_column, foreign_key)
             end
-            if foreign_key_was && model.respond_to?(:decrement_counter)
-              model.decrement_counter(cache_column, foreign_key_was)
+
+            if foreign_key_was && model_was.respond_to?(:decrement_counter)
+              model_was.decrement_counter(cache_column, foreign_key_was)
             end
           end
         end