Expire the stats cache once per day

When we first started caching the stats, generating them was a process
that took several minutes, so we never expired the cache.

However, there have been cases where we run into issues where the stats
shown on the screen were outdated. That's why we introduced a task to
manually expire the cache.

But now, generating the stats only takes a few seconds, so we can
automatically expire them every day, remove all the logic needed to
manually expire them, and get rid of most of the issues related to the
cache being outdated.

We're expiring them every day because it's the same day we were doing in
public stats (which we removed in commit 631b48f58), only we're using
`expires_at:` to set the expiration time, in order to simplify the code.

Note that, in the test, we're using `travel_to(time)` so the test passes
even when it starts an instant before midnight. We aren't using
`:with_frozen_time` because, in similar cases (although not in this
case, but I'm not sure whether that's intentional), `travel_to` shows
this error:

> Calling `travel_to` with a block, when we have previously already made
> a call to `travel_to`, can lead to confusing time stubbing.

app/models/budget.rb

@@ -1,7 +1,6 @@
 class Budget < ApplicationRecord
   include Measurable
   include Sluggable
-  include StatsVersionable
   include Reportable
   include Imageable
 

app/models/budget/stats.rb

@@ -193,6 +193,6 @@ class Budget::Stats
     stats_cache(*stats_methods)
 
     def full_cache_key_for(key)
-      "budgets_stats/#{budget.id}/#{phases.join}/#{key}/#{version}"
+      "budgets_stats/#{budget.id}/#{phases.join}/#{key}"
     end
 end

app/models/concerns/statisticable.rb

@@ -149,10 +149,6 @@ module Statisticable
     (fraction * 100.0 / total).round(3)
   end
 
-  def version
-    "v#{resource.find_or_create_stats_version.updated_at.to_i}"
-  end
-
   def advanced?
     resource.advanced_stats_enabled?
   end
@@ -231,6 +227,6 @@ module Statisticable
     end
 
     def stats_cache(key, &)
-      Rails.cache.fetch(full_cache_key_for(key), &)
+      Rails.cache.fetch(full_cache_key_for(key), expires_at: Date.current.end_of_day, &)
     end
 end

app/models/concerns/stats_versionable.rb

@@ -1,11 +0,0 @@
-module StatsVersionable
-  extend ActiveSupport::Concern
-
-  included do
-    has_one :stats_version, as: :process, inverse_of: :process
-  end
-
-  def find_or_create_stats_version
-    stats_version || create_stats_version
-  end
-end

app/models/poll.rb

@@ -7,7 +7,6 @@ class Poll < ApplicationRecord
   include Notifiable
   include Searchable
   include Sluggable
-  include StatsVersionable
   include Reportable
   include SDG::Relatable
 

app/models/poll/stats.rb

@@ -122,6 +122,6 @@ class Poll::Stats
     stats_cache(*stats_methods)
 
     def full_cache_key_for(key)
-      "polls_stats/#{poll.id}/#{key}/#{version}"
+      "polls_stats/#{poll.id}/#{key}"
     end
 end

app/models/stats_version.rb

@@ -1,5 +0,0 @@
-class StatsVersion < ApplicationRecord
-  validates :process, presence: true
-
-  belongs_to :process, polymorphic: true
-end

db/migrate/20240408154814_drop_stats_versions.rb

@@ -0,0 +1,12 @@
+class DropStatsVersions < ActiveRecord::Migration[7.0]
+  def change
+    drop_table :stats_versions, id: :serial do |t|
+      t.string :process_type
+      t.integer :process_id
+      t.datetime :created_at, precision: nil, null: false
+      t.datetime :updated_at, precision: nil, null: false
+
+      t.index ["process_type", "process_id"], name: "index_stats_versions_on_process_type_and_process_id"
+    end
+  end
+end

db/schema.rb

@@ -1515,14 +1515,6 @@ ActiveRecord::Schema[7.0].define(version: 2024_04_24_013913) do
     t.string "locale"
   end
 
-  create_table "stats_versions", id: :serial, force: :cascade do |t|
-    t.string "process_type"
-    t.integer "process_id"
-    t.datetime "created_at", precision: nil, null: false
-    t.datetime "updated_at", precision: nil, null: false
-    t.index ["process_type", "process_id"], name: "index_stats_versions_on_process_type_and_process_id"
-  end
-
   create_table "taggings", id: :serial, force: :cascade do |t|
     t.integer "tag_id"
     t.string "taggable_type"

lib/tasks/stats.rake

@@ -1,10 +0,0 @@
-namespace :stats do
-  desc "Expires stats cache"
-  task expire_cache: :environment do
-    Tenant.run_on_each do
-      [Budget, Poll].each do |model_class|
-        model_class.find_each { |record| record.find_or_create_stats_version.touch }
-      end
-    end
-  end
-end

spec/models/poll/stats_spec.rb

@@ -299,32 +299,4 @@ describe Poll::Stats do
       end
     end
   end
-
-  describe "#version", :with_frozen_time do
-    context "record with no stats" do
-      it "returns a string based on the current time" do
-        expect(stats.version).to eq "v#{Time.current.to_i}"
-      end
-
-      it "doesn't overwrite the timestamp when called multiple times" do
-        time = Time.current
-
-        expect(stats.version).to eq "v#{time.to_i}"
-
-        unfreeze_time
-
-        travel_to 2.seconds.from_now do
-          expect(stats.version).to eq "v#{time.to_i}"
-        end
-      end
-    end
-
-    context "record with stats" do
-      before { poll.create_stats_version(updated_at: 1.day.ago) }
-
-      it "returns the version of the existing stats" do
-        expect(stats.version).to eq "v#{1.day.ago.to_i}"
-      end
-    end
-  end
 end

spec/models/statisticable_spec.rb

@@ -4,6 +4,12 @@ describe Statisticable do
   before do
     dummy_stats = Class.new do
       include Statisticable
+      attr_accessor :total
+      stats_cache :total
+
+      def full_cache_key_for(key)
+        "dummy_stats/#{object_id}/#{key}"
+      end
 
       def participants
         User.all
@@ -221,4 +227,28 @@ describe Statisticable do
       end.each(&:join)
     end
   end
+
+  describe "cache" do
+    it "expires the cache at the end of the day", :with_cache do
+      time = Time.current
+
+      travel_to(time) do
+        stats.total = 6
+
+        expect(stats.total).to eq 6
+
+        stats.total = 7
+
+        expect(stats.total).to eq 6
+      end
+
+      travel_to(time.end_of_day) do
+        expect(stats.total).to eq 6
+      end
+
+      travel_to(time.end_of_day + 1.second) do
+        expect(stats.total).to eq 7
+      end
+    end
+  end
 end

spec/models/stats_version_spec.rb

@@ -1,13 +0,0 @@
-require "rails_helper"
-
-describe StatsVersion do
-  describe "validations" do
-    it "is valid with a process" do
-      expect(StatsVersion.new(process: Budget.new)).to be_valid
-    end
-
-    it "is not valid without a process" do
-      expect(StatsVersion.new(process: nil)).not_to be_valid
-    end
-  end
-end