Second and Third day retention calculator from seen_at and first_seen_at in MongoDB Aggregation FW

mongo_daily_retention.rb

User.disk_aggregate([
  { '$match' => { '$and' => [
    { 'attrs.first_seen_at' => { '$gte' => DateTime.parse('2015-06-01') } },
    { 'attrs.first_seen_at' => { '$lte' => DateTime.parse('2015-06-30 23:59') } }] } },
  { '$project' => {
    days_seen_at: { '$map'=> {
      input: '$attrs.seen_at', 
      as: 'date', 
      in: { '$dayOfYear' => '$$date' } } },
    fsa: { '$dayOfYear' => '$attrs.first_seen_at' },
    uid: '$attrs.unique_id',
    _id: 0
  } },
  { '$unwind' => '$days_seen_at' },
  { '$project' => {
    uid: 1,
    second_day: { '$let' => {
      vars: { day_fsa: { '$add' => ['$fsa', 1] } },
      'in' => { '$cond' => {
        'if' => { '$eq' => ['$$day_fsa', '$days_seen_at'] }, then: 1, else: 0
      } } } },
    third_day: { '$let' => {
      vars: { day_fsa: { '$add' => ['$fsa', 2] } },
      'in' => { '$cond' => {
        'if' => { '$eq' => ['$$day_fsa', '$days_seen_at'] }, then: 1, else: 0
      } }
    } } } },
  { '$group' => {
    _id: '$uid',
    second_day: { '$max' => '$second_day' },
    third_day: { '$max' => '$third_day' }
  } },
  { '$group' => {
    _id: nil,
    second_day: { '$sum' => '$second_day' },
    third_day: { '$sum' => '$third_day' }
  }}
])