# File lib/mongoid/relations/referenced/many_to_many.rb, line 27
        def <<(*args)
          batched do
            [].tap do |ids|
              args.flatten.each do |doc|
                next unless doc
                append(doc)
                if persistable? || _creating?
                  ids.push(doc.id)
                  doc.save
                else
                  base.send(metadata.foreign_key).push(doc.id)
                  base.synced[metadata.foreign_key] = false
                end
              end
              if persistable? || _creating?
                base.push_all(metadata.foreign_key, ids)
                base.synced[metadata.foreign_key] = false
              end
            end
          end
        end