1 files changed, 36 insertions, 0 deletions
diff --git a/activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb b/activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb
new file mode 100644
index 0000000000..9ae452e7a1
--- /dev/null
+++ b/activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb
@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+module ActiveRecord
+  module Associations
+    # = Active Record Belongs To Polymorphic Association
+    class BelongsToPolymorphicAssociation < BelongsToAssociation #:nodoc:
+      def klass
+        type = owner[reflection.foreign_type]
+        type.presence && type.constantize
+      end
+
+      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
+        end
+
+        def inverse_reflection_for(record)
+          reflection.polymorphic_inverse_of(record.class)
+        end
+
+        def raise_on_type_mismatch!(record)
+          # A polymorphic association cannot have a type mismatch, by definition
+        end
+
+        def stale_state
+          foreign_key = super
+          foreign_key && [foreign_key.to_s, owner[reflection.foreign_type].to_s]
+        end
+    end
+  end
+end