Remove Cron job to generate stats

Since now generating stats (assuming the results aren't in the cache)
only takes a few seconds even when there are a hundred thousand
participants, as opposed to the several minutes it took to generate them
when we introduced the Cron job, we can simply generate the stats during
the first request to the stats page.

Note that, in order to avoid creating a temporary table when the stats
are cached, we're making sure we only create this table when we need to.
Otherwise, we could spend up to 1 second on every request to the stats
page creating a table that isn't going to be used.

Also note we're using an instance variable to check whether we're
creating a table; I tried to use `table_exists?`, but it didn't work. I
wonder whether `table_exists?` doesn't detect temporary tables.

app/controllers/budgets/stats_controller.rb

@@ -8,7 +8,7 @@ module Budgets
 
     def show
       authorize! :read_stats, @budget
-      @stats = Budget::Stats.new(@budget)
+      @stats = Budget::Stats.new(@budget).tap(&:generate)
       @headings = @budget.headings.sort_by_name
     end
 

app/controllers/polls_controller.rb

@@ -23,7 +23,7 @@ class PollsController < ApplicationController
   end
 
   def stats
-    @stats = Poll::Stats.new(@poll)
+    @stats = Poll::Stats.new(@poll).tap(&:generate)
   end
 
   def results

app/models/concerns/statisticable.rb

@@ -48,10 +48,9 @@ module Statisticable
 
   def generate
     User.transaction do
-      create_participants_table
-
       begin
         define_singleton_method :participants do
+          create_participants_table unless participants_table_created?
           participants_class.all
         end
 
@@ -176,10 +175,12 @@ module Statisticable
       )
       User.connection.add_index participants_table_name, :date_of_birth
       User.connection.add_index participants_table_name, :geozone_id
+      @participants_table_created = true
     end
 
     def drop_participants_table
       User.connection.drop_table(participants_table_name, if_exists: true, temporary: true)
+      @participants_table_created = false
     end
 
     def participants_table_name
@@ -190,6 +191,10 @@ module Statisticable
       @participants_class ||= Class.new(User).tap { |klass| klass.table_name = participants_table_name }
     end
 
+    def participants_table_created?
+      @participants_table_created.present?
+    end
+
     def total_participants_with_gender
       @total_participants_with_gender ||= participants.where.not(gender: nil).distinct.count
     end

config/schedule.rb

@@ -27,10 +27,6 @@ every 1.day, at: "5:00 am" do
   rake "-s sitemap:refresh:no_ping"
 end
 
-every 2.hours do
-  rake "-s stats:generate"
-end
-
 every 1.day, at: "1:00 am", roles: [:cron] do
   rake "files:remove_old_cached_attachments"
 end

lib/tasks/stats.rake

@@ -1,23 +1,4 @@
 namespace :stats do
-  desc "Generates stats which are not cached yet"
-  task generate: :environment do
-    ApplicationLogger.new.info "Updating budget and poll stats"
-
-    Tenant.run_on_each do
-      admin_ability = Ability.new(Administrator.first.user)
-
-      Budget.accessible_by(admin_ability, :read_stats).find_each do |budget|
-        Budget::Stats.new(budget).generate
-        print "."
-      end
-
-      Poll.accessible_by(admin_ability, :stats).find_each do |poll|
-        Poll::Stats.new(poll).generate
-        print "."
-      end
-    end
-  end
-
   desc "Expires stats cache"
   task expire_cache: :environment do
     Tenant.run_on_each do
@@ -26,7 +7,4 @@ namespace :stats do
       end
     end
   end
-
-  desc "Deletes stats cache and generates it again"
-  task regenerate: [:expire_cache, :generate]
 end