Usually you can use attr_accessor to define virtual attribute.
You could even use attr_accessor_with_default to set default value.
But in any case you are unable to typecast values.
Let's assume you receive data from form submit.
> params[:user].inspect
=> {'enabled' => 'true', 'priority' => '0.5'}
Having attributes defined as table columns values would be actual true and 0.5.
But with attr_accessor fields will still be strings.
This is what virtual_attribute handles.
Inspired with virtual_attribute gem by Philip Roberts I tried to use ActiveRecord to typecast.
class User < ActiveRecord::Base
virtual_attribute :enabled, :type => :boolean, :default => false
virtual_attribute :priority, :type => :float, :default => 1.0
end