Grouping results and summarizing a field in one query with Mongoid?

I'm trying to execute a more or less advanced query with Mongoid that basically gets metrics for a date range, groups them by day and then summarizes the values for each day, it should also tell me how many entries there are for each day.

I highly doubt this can be done with the active record part of Mongoid, but I don't know how to execute queries on the mongo driver directly.

My model:

class Metric
  include Mongoid::Document

  field :id_session,    :type => Integer
  field :metric,        :type => Integer
  field :value,         :type => Integer
  field :date,          :type => Date
  field :time,          :type => Time

  validates_numericality_of :value
  validates_presence_of :id_session, :metric, :value

  before_create :set_date

  def set_date
    self.time   = Time.now
    self.date   = Date.now
  end

end

I've been able to get the results groupe开发者_Python百科d by date simply by using Metric.distinct(:date), but I don't know how to do a sum and count of those results as I can't use that method on the results.

Any ideas? I prefer to stay within the Mongoid active record methods but if anyone knows how I can execute queries directly on the MongoDB driver that would help too.

Thanks!

Managed to get it working

    result = Metric.collection.group(
  ['date'] , nil, {:count => 0, :value => 0}, "function(x, y) { y.count++; y.value += x.value }"
)

Credits go to the answer on this page