Define ActiveRecord::Attribute::Null#type_cast

Using ActiveRecord::Base.attribute to declare an attribute with a default value on a model where the attribute is not backed by the database would raise a NotImplementedError when model.save is called.

The error originates from https://github.com/rails/rails/blob/59d252196b36f6afaafd231756d69ea21537cf5d/activerecord/lib/active_record/attribute.rb#L84.
This is called from https://github.com/rails/rails/blob/59d252196b36f6afaafd231756d69ea21537cf5d/activerecord/lib/active_record/attribute.rb#L46 on an ActiveRecord::Attribute::Null object.

This commit corrects the behavior by implementing ActiveRecord::Attribute::Null#type_cast.

With ActiveRecord::Attribute::Null#type_cast defined, ActiveRecord::Attribute::Null#value (https://github.com/rails/rails/blob/59d252196b36f6afaafd231756d69ea21537cf5d/activerecord/lib/active_record/attribute.rb#L173..L175) can be replaced with its super method (https://github.com/rails/rails/blob/59d252196b36f6afaafd231756d69ea21537cf5d/activerecord/lib/active_record/attribute.rb#L36..L40).

fixes #24979

1 files changed, 9 insertions, 0 deletions

diff --git a/activerecord/test/cases/attributes_test.rb b/activerecord/test/cases/attributes_test.rb
index 2bebbfa205..48ba7a63d5 100644
--- a/activerecord/test/cases/attributes_test.rb
+++ b/activerecord/test/cases/attributes_test.rb
@@ -63,6 +63,15 @@ module ActiveRecord
       end
     end
 
+    test "model with nonexistent attribute with default value can be saved" do
+      klass = Class.new(OverloadedType) do
+        attribute :non_existent_string_with_default, :string, default: 'nonexistent'
+      end
+
+      model = klass.new
+      assert model.save
+    end
+
     test "changing defaults" do
       data = OverloadedType.new
       unoverloaded_data = UnoverloadedType.new