Mongoid：聚合框架在范围内有效？

Question

我有以下在 ruby​​ 中工作的方法。基本上，它使用聚合框架在活跃的美国交易中选择按日销售量排序的前 k 笔交易。Deal 是 MongoDB 中的一个集合。

  def top_daily_quantity(k)
    match = { "$match" =>
                  {
                      country: "US",
                      active: true
                  }
    }
    project = {
        "$project" => {
            _id: 1,
            sold_quantity: 1,
            days: {"$divide" => [{"$subtract" => [Time.zone.now, "$start_at"]}, 3600*24*1000]}, #note days is in float
            daily_quantity: {
                "$divide" => [
                    {"$multiply" => ["$sold_quantity", 3600*24*1000]},
                    {"$subtract" => [Time.zone.now, "$start_at"]} # this difference is in ms
                ]
            }
        }
    }
    sort = { "$sort" => { daily_quantity: -1 } }
    limit = { "$limit" =>  k}
    collection.aggregate([match, project, sort, limit])
  end

问题是我想将其更改为范围，以便我可以将许多范围链接在一起，以灵活地进行复杂查询。我只是改成下面的格式：

scope :top_k, lambda { |k|
    ... # the same method body as uppoer
       }

然后我得到以下错误：

1.9.3-p385 :001 > DS::Deal.top_k(3)

NoMethodError：未定义的方法to_criteria' for #<Array:0x007fb96f35e3c0> from /Users/x/.rvm/gems/ruby-1.9.3-p385/gems/mongoid-3.1.5/lib/mongoid/criteria.rb:292:in合并！' 来自 /Users/x/.rvm/gems/ruby-1.9.3-p385/gems/mongoid-3.1.5/lib/mongoid/criteria.rb:277:in merge' from /Users/x/.rvm/gems/ruby-1.9.3-p385/gems/mongoid-3.1.5/lib/mongoid/scoping.rb:305:intop_k' from (irb):1 from /Users/ x/.rvm/gems/ruby-1.9.3-p385/gems/railties-3.2.14/lib/rails/commands/console.rb:47:in start' from /Users/x/.rvm/gems/ruby-1.9.3-p385/gems/railties-3.2.14/lib/rails/commands/console.rb:8:instart' from /Users/x/.rvm/gems/ruby -1.9.3-p385/gems/railties-3.2.14/lib/rails/commands.rb:41:in <top (required)>' from script/rails:6:inrequire' from script/rails:6:in `'

那是因为最后一行返回一个数组，它打破了作用域的要求。从http://mongoid.org/en/mongoid/docs/querying.html看来，我只能为此使用 map reduce，但我想在这里查看是否遗漏了什么。