Demonstrate MongoDB aggregation with the Ruby Mongoid gem.

mongo_aggregation.rb

require 'mongoid'
require 'pp'

Mongoid.configure do |config|
  config.allow_dynamic_fields = false
  config.connect_to 'mongo_test', { consistency: :strong }
end

Mongoid.default_session.drop

class Event
  include Mongoid::Document
  include Mongoid::Timestamps
  has_many :activities, as: :subject
end

class Activity
  include Mongoid::Document
  include Mongoid::Timestamps
  belongs_to :subject, polymorphic: true
  field :name, type: String
  field :action, type: Symbol
end

event = Event.create!
event2 = Event.create!
event3 = Event.create!
Activity.create! subject: event, created_at: 2.seconds.ago, name: 'a', action: :tab
Activity.create! subject: event, created_at: 1.second.ago, name: 'b', action: :share
Activity.create! subject: event, name: 'c', action: :follow
Activity.create! subject: event2, name: 'd', action: :share
Activity.create! subject: event2, name: 'e', action: :follow, created_at: 5.seconds.ago
Activity.create! subject: event2, name: 'f', action: :comment
Activity.create! subject: event3, name: 'g', action: :share

puts Activity.count
puts Activity.all.to_a.inspect
puts
pp Activity.collection.aggregate([
    {
      '$sort' => {'created_at' => 1}
    },
    {
      '$match' => {
        'action' => {
          '$nin' => [:comment, :favorite]
        }
      }
    },
    {
      '$project' => {
        'subject' => {
          'subject_id' => '$subject_id',
          'subject_type' => '$subject_type'
        },
        'name' => 1,
        'id' => '$_id'
      }
    },
    {
      '$group' => {
        '_id' => '$subject',
        'name' => {'$first' => '$name'},
        'others_count' => {'$sum' => 1},
        'id' => {'$first' => '$id'}
      }
    },
    {
      '$project' => {
        '_id' => '$id',
        'name' => 1,
        'others_count' => {'$subtract' => ['$others_count', 1]}
      }
    }
  ])

# [{"_id"=>"51b61350cfe415b9b400000a", "name"=>"g", "others_count"=>0},
#  {"_id"=>"51b61350cfe415b9b4000004", "name"=>"a", "others_count"=>2},
#  {"_id"=>"51b61350cfe415b9b4000008", "name"=>"e", "others_count"=>1}]