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

activerecord/lib/active_record/attribute.rb

@@ -205,6 +205,8 @@ def with_value_from_database(value)
     end
 
     class Uninitialized < Attribute # :nodoc:
+      UNINITIALIZED_ORIGINAL_VALUE = Object.new
+
       def initialize(name, type)
         super(name, nil, type)
       end
@@ -215,6 +217,10 @@ def value
         end
       end
 
+      def original_value
+        UNINITIALIZED_ORIGINAL_VALUE
+      end
+
       def value_for_database
       end
 

activerecord/test/cases/dirty_test.rb

@@ -734,6 +734,17 @@ def catchphrase
     assert_equal "arr", pirate.catchphrase
   end
 
+  test "attributes assigned but not selected are dirty" do
+    person = Person.select(:id).first
+    refute person.changed?
+
+    person.first_name = "Sean"
+    assert person.changed?
+
+    person.first_name = nil
+    assert person.changed?
+  end
+
   private
     def with_partial_writes(klass, on = true)
       old = klass.partial_writes?