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, 33 insertions, 0 deletions

diff --git a/activerecord/test/cases/serialized_attribute_test.rb b/activerecord/test/cases/serialized_attribute_test.rb
index 6056156698..846be857d0 100644
--- a/activerecord/test/cases/serialized_attribute_test.rb
+++ b/activerecord/test/cases/serialized_attribute_test.rb
@@ -295,4 +295,37 @@ class SerializedAttributeTest < ActiveRecord::TestCase
     topic.update_attribute :content, nil
     assert_equal [topic], Topic.where(content: nil)
   end
+
+  def test_mutation_detection_does_not_double_serialize
+    coder = Object.new
+    def coder.dump(value)
+      return if value.nil?
+      value + " encoded"
+    end
+    def coder.load(value)
+      return if value.nil?
+      value.gsub(" encoded", "")
+    end
+    type = Class.new(ActiveModel::Type::Value) do
+      include ActiveModel::Type::Helpers::Mutable
+
+      def serialize(value)
+        return if value.nil?
+        value + " serialized"
+      end
+
+      def deserialize(value)
+        return if value.nil?
+        value.gsub(" serialized", "")
+      end
+    end.new
+    model = Class.new(Topic) do
+      attribute :foo, type
+      serialize :foo, coder
+    end
+
+    topic = model.create!(foo: "bar")
+    topic.foo
+    refute topic.changed?
+  end
 end