Merge pull request #29018 from willbryant/missing_attributes_after_save

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

activemodel/lib/active_model/attribute.rb

@@ -233,6 +233,10 @@ module ActiveModel
           false
         end
 
+        def forgetting_assignment
+          dup
+        end
+
         def with_type(type)
           self.class.new(name, type)
         end

activerecord/test/cases/dirty_test.rb

@@ -771,6 +771,13 @@ class DirtyTest < ActiveRecord::TestCase
     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")