# File lib/mongoid/relations/embedded/one.rb, line 37
        def substitute(replacement)
          tap do |proxy|
            if _assigning?
              base.atomic_unsets.push(proxy.atomic_path)
            else
              proxy.delete if persistable?
            end
            proxy.unbind_one
            return nil unless replacement
            proxy.target = replacement
            proxy.bind_one
          end
        end