2 files changed, 40 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
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