Ensure that records with unselected fields can be updated

As part of refactoring mutation detection to be more performant, we
introduced the concept of `original_value` to `Attribute`. This was not
overridden in `Attribute::Uninitialized` however, so assigning ot an
uninitialized value and calling `.changed?` would raise
`NotImplementedError`.

We are using a sentinel value rather than checking the result of
`original_attribute.initialized?` in `changed?` because `original_value`
might go through more than one node in the tree.

Fixes #25228

1 files changed, 6 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/attribute.rb b/activerecord/lib/active_record/attribute.rb
index cacde0f6fd..9530f134d0 100644
--- a/activerecord/lib/active_record/attribute.rb
+++ b/activerecord/lib/active_record/attribute.rb
@@ -205,6 +205,8 @@ module ActiveRecord
     end
 
     class Uninitialized < Attribute # :nodoc:
+      UNINITIALIZED_ORIGINAL_VALUE = Object.new
+
       def initialize(name, type)
         super(name, nil, type)
       end
@@ -215,6 +217,10 @@ module ActiveRecord
         end
       end
 
+      def original_value
+        UNINITIALIZED_ORIGINAL_VALUE
+      end
+
       def value_for_database
       end