Fix false positive mutation detection when JSON is used with serialize

When looking for mutation, we compare the serialized version of the
value to the before_type_cast form. `Type::Serialized` was breaking this
contract by passing the already serialized attribute to the subtype's
mutation detection. This never manifested previously, as all mutable
subtypes either didn't do anything in their `serialize` method, or had a
way to detect double serialization (e.g. `is_a?(String)`). However, now
that JSON types can handle string primitives, we need to avoid double
serialization.

Fixes #24993.

1 files changed, 7 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/type/serialized.rb b/activerecord/lib/active_record/type/serialized.rb
index 4ff0740cfb..a3a5241780 100644
--- a/activerecord/lib/active_record/type/serialized.rb
+++ b/activerecord/lib/active_record/type/serialized.rb
@@ -32,7 +32,7 @@ module ActiveRecord
 
       def changed_in_place?(raw_old_value, value)
         return false if value.nil?
-        raw_new_value = serialize(value)
+        raw_new_value = encoded(value)
         raw_old_value.nil? != raw_new_value.nil? ||
           subtype.changed_in_place?(raw_old_value, raw_new_value)
       end
@@ -52,6 +52,12 @@ module ActiveRecord
       def default_value?(value)
         value == coder.load(nil)
       end
+
+      def encoded(value)
+        unless default_value?(value)
+          coder.dump(value)
+        end
+      end
     end
   end
 end