I see! But isn't the default behaviour (that is, when no coder is specified) to use YAML coder?

As on lines 169~173 of store.rb:

if coder_or_class_name.respond_to?(:load) && coder_or_class_name.respond_to?(:dump)
    coder_or_class_name
else
     ActiveRecord::Coders::YAMLColumn.new(coder_or_class_name || Object)
end

Please correct me if I'm wrong (: