Add graphs and retention metrics to admin dashboard (#16829)

app/lib/admin/metrics/retention.rb

@@ -0,0 +1,67 @@
+# frozen_string_literal: true
+
+class Admin::Metrics::Retention
+  class Cohort < ActiveModelSerializers::Model
+    attributes :period, :frequency, :data
+  end
+
+  class CohortData < ActiveModelSerializers::Model
+    attributes :date, :percent, :value
+  end
+
+  def initialize(start_at, end_at, frequency)
+    @start_at  = start_at&.to_date
+    @end_at    = end_at&.to_date
+    @frequency = %w(day month).include?(frequency) ? frequency : 'day'
+  end
+
+  def cohorts
+    sql = <<-SQL.squish
+      SELECT axis.*, (
+        WITH new_users AS (
+          SELECT users.id
+          FROM users
+          WHERE date_trunc($3, users.created_at)::date = axis.cohort_period
+        ),
+        retained_users AS (
+          SELECT users.id
+          FROM users
+          INNER JOIN new_users on new_users.id = users.id
+          WHERE date_trunc($3, users.current_sign_in_at) >= axis.retention_period
+        )
+        SELECT ARRAY[count(*), (count(*) + 1)::float / (SELECT count(*) + 1 FROM new_users)] AS retention_value_and_percent
+        FROM retained_users
+      )
+      FROM (
+        WITH cohort_periods AS (
+          SELECT generate_series(date_trunc($3, $1::timestamp)::date, date_trunc($3, $2::timestamp)::date, ('1 ' || $3)::interval) AS cohort_period
+        ),
+        retention_periods AS (
+          SELECT cohort_period AS retention_period FROM cohort_periods
+        )
+        SELECT *
+        FROM cohort_periods, retention_periods
+        WHERE retention_period >= cohort_period
+      ) as axis
+    SQL
+
+    rows = ActiveRecord::Base.connection.select_all(sql, nil, [[nil, @start_at], [nil, @end_at], [nil, @frequency]])
+
+    rows.each_with_object([]) do |row, arr|
+      current_cohort = arr.last
+
+      if current_cohort.nil? || current_cohort.period != row['cohort_period']
+        current_cohort = Cohort.new(period: row['cohort_period'], frequency: @frequency, data: [])
+        arr << current_cohort
+      end
+
+      value, percent = row['retention_value_and_percent'].delete('{}').split(',')
+
+      current_cohort.data << CohortData.new(
+        date: row['retention_period'],
+        percent: percent.to_f,
+        value: value.to_s
+      )
+    end
+  end
+end