Mongoid: does the aggregation structure work in scope?

Question

I have the following method working in ruby. He mainly uses the aggregation structure to select the best trades ordered by the daily quantity sold in active trades with the United States. A deal is a collection in MongoDB.

  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

I want to change it into scope so that I can combine many scopes for a complex query with flexibility. I just changed the format below:

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

Then I got the errors below:

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

NoMethodError: undefined 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 merge! '    /Users/x/.rvm/gems/ruby-1.9.3-p385/gems/mongoid-3.1.5/lib/mongoid/criteria.rb:77:in merge' from /Users/x/.rvm/gems/ruby-1.9.3-p385/gems/mongoid-3.1.5/lib/mongoid/scoping.rb:305:in top_k '    (irb): 1    /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:in start '    /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:in require '    script/rails: 6: in `'

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