# File lib/mongoid/persistence/atomic/add_to_set.rb, line 19
        def persist
          prepare do
            document[field] = [] unless document[field]
            values = document.send(field)
            values.push(value) unless values.include?(value)
            values.tap do
              if document.persisted?
                collection.update(document.atomic_selector, operation("$addToSet"), options)
                document.remove_change(field)
              end
            end
          end
        end