fix the dirty tracking code's save hook overwriting missing attribute…

…s with initialized-to-nil attributes. fixes #29017.

activerecord/lib/active_record/attribute.rb

@@ -231,6 +231,10 @@ def initialized?
           false
         end
 
+        def forgetting_assignment
+          dup
+        end
+
         def with_type(type)
           self.class.new(name, type)
         end

activerecord/test/cases/dirty_test.rb

@@ -775,6 +775,13 @@ def catchphrase
     assert person.changed?
   end
 
+  test "attributes not selected are still missing after save" do
+    person = Person.select(:id).first
+    assert_raises(ActiveModel::MissingAttributeError) { person.first_name }
+    assert person.save # calls forget_attribute_assignments
+    assert_raises(ActiveModel::MissingAttributeError) { person.first_name }
+  end
+
   test "saved_change_to_attribute? returns whether a change occurred in the last save" do
     person = Person.create!(first_name: "Sean")