Mongo DB hierarchical aggregation

base_collection.rb

class BaseCollection
  include Mongoid::Document
  
  aggregate(:daily,
            map: %Q(function() {
              var key = {
                year_day: this.year_day,
                year_week: this.year_week,
                year_month: this.year_month
              };
              var value = {
                count: 1
              };
              emit(key, value);
            }),
            reduce: %Q(function(key, values) {
              var result = {
                count: 0
              };
              values.forEach(function(value){
                result.count += value.count;
              });
              return result;
            }))
end

daily.rb

class Daily
  include Mongoid::Document
  
  aggregate(:weekly,
            map: %Q(function() {
              var key = {
                year_week: this._id.year_week,
                year_month: this._id.year_month
              };
              var value = {
                count: 1
              };
              emit(key, value);
            }),
            reduce: %Q(function(key, values) {
              var result = {
                count: 0
              };
              values.forEach(function(value){
                result.count += value.count;
              });
              return result;
            }))
end

finalize.rb

aggregate(

  # ...

  finalize: %Q(function(key, value) {
              value.aggregated_at = new Date;
              return value;
            }))

record.rb

class Record < AR::Base
  
  # my dear SQL table exports itself to a MongoDB collection
  
  def self.export
    all.each {|record|
      date = reccord.created_at
      document = {}
      
      document[:year_day] = date.strftime('%Y-%m-%d') # also %j
      document[:year_week] = date.strftime('%Y-%W')
      document[:year_month] = date.strftime('%Y-%m')
      
      # ... whatever makes your document a 'nice document' ...
      
      BaseCollection.create(document)
    }
  end
end

task.rb

task aggregation_round: :environment do
  Record.export
  BaseCollection.aggregate_to(:daily)
  Daily.aggregate_to(:weekly)
  Weekly.aggregate_to(:monthly)
end

weekly.rb

class Weekly
  include Mongoid::Document
  
  aggregate(:monthly,
            map: %Q(function() {
              var key = {
                year_month: this._id.year_month
              };
              var value = {
                count: 1
              };
              emit(key, value);
            }),
            reduce: %Q(function(key, values) {
              var result = {
                count: 0
              };
              values.forEach(function(value){
                result.count += value.count;
              });
              return result;
            }))
end