Aggregate service prototype

AggregateService.rb

module AggregateService
  def self.aggregate(collection, &block)
    agg = Aggregate.new(collection, &block)
    agg
  end

  module AggregateStages
    def match(&block)
      @stages << {
          '$match' => Match.new(&block).formatted
      }
    end

    class Match

      def initialize(&constructor)
        @field_list = {}
        instance_eval(&constructor)
      end

      def field(key, opts)
        @field_list[key] = opts
      end

      def formatted
        @field_list
      end
    end

    def group(key, &block)
      @stages << {
        '$group' => Group.new(key, &block).formatted
      }
    end

    class Group
      def initialize(key, &constructor)
        @field_list = {}
        @key = key
        instance_eval(&constructor)
      end

      def field(key, opts)
        @field_list[key] = opts
      end

      def formatted
        {
          "_id" => @key
        }.merge(@field_list)
      end

    end

    def add_field

    end

    class AddField

      def initialize(&constructor)
        @field_list = {}
        instance_eval(&constructor)
      end

      def field(key, opts)
        @field_list[key] = opts
      end
    end
  end

  class Aggregate
    include AggregateStages

    def initialize(collection, &constructor)
      @collection = collection
      @stages = []
      instance_eval(&constructor)
    end

    def results
      @collection.aggregate(@stages)
    end

  end
end

example_usage.rb

AggregateService.aggregate(:collection_name) do
  match do
    field 'sensor_id', eq: 'test'
    field 'date', gte: '2010-10-10'
  end
  add_field do
    field 'temp_diff', '$abs' => { '$subtract' => ['$temp', '$ambient_temp'] }
  end
  group 'sensor_id' do
    field 'min_temp', '$max' => '$temp'
    field 'max_temp', '$min' => '$temp'
  end
end