class Porkchop
  include Mongoid::Document

  default_scope where(:default => true)

  scope :tasty, where(:tasty => true)
end

Porkchop.unscoped.tasty
=> #<Mongoid::Criteria
  selector: {:default=>true, :tasty=>true},
  options:  {},
  class:    Porkchop,
  embedded: false>