Add table to store stats versions

We need a way to manually expire the cache for a budget or poll without
expiring the cache of every budget or poll.

Using the `updated_at` column would be dangerous because most of the
times we update a budget or a poll, we don't need to regenerate their
stats.

We've considered adding a `stats_updated_at` column to each of these
tables. However, in that case we would also need to add a similar column
in the future to every process type whose stats we want to generate.

app/models/budget.rb

@@ -2,6 +2,7 @@ class Budget < ApplicationRecord
 
   include Measurable
   include Sluggable
+  include StatsVersionable
 
   translates :name, touch: true
   include Globalizable

app/models/budget/stats.rb

@@ -178,6 +178,6 @@ class Budget::Stats
     stats_cache :voters, :participants, :authors, :balloters, :poll_ballot_voters
 
     def stats_cache(key, &block)
-      Rails.cache.fetch("budgets_stats/#{budget.id}/#{phases.join}/#{key}/v10", &block)
+      Rails.cache.fetch("budgets_stats/#{budget.id}/#{phases.join}/#{key}/#{version}", &block)
     end
 end

app/models/concerns/statisticable.rb

@@ -88,6 +88,10 @@ module Statisticable
       PercentageCalculator.calculate(fraction, total)
     end
 
+    def version
+      "v#{resource.find_or_create_stats_version.updated_at.to_i}"
+    end
+
     private
 
       def base_stats_methods

app/models/concerns/stats_versionable.rb

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

app/models/poll.rb

@@ -6,6 +6,7 @@ class Poll < ApplicationRecord
   include ActsAsParanoidAliases
   include Notifiable
   include Sluggable
+  include StatsVersionable
 
   translates :name,        touch: true
   translates :summary,     touch: true

app/models/poll/stats.rb

@@ -107,7 +107,7 @@ class Poll::Stats
     stats_cache :participants, :voters, :recounts
 
     def stats_cache(key, &block)
-      Rails.cache.fetch("polls_stats/#{poll.id}/#{key}/v12", &block)
+      Rails.cache.fetch("polls_stats/#{poll.id}/#{key}/#{version}", &block)
     end
 
 end

app/models/stats_version.rb

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

db/migrate/20190408133956_create_stats_versions.rb

@@ -0,0 +1,8 @@
+class CreateStatsVersions < ActiveRecord::Migration[4.2]
+  def change
+    create_table :stats_versions do |t|
+      t.references :process, polymorphic: true, index: true
+      t.timestamps null: false
+    end
+  end
+end

db/schema.rb

@@ -1339,6 +1339,14 @@ ActiveRecord::Schema.define(version: 20190411090023) do
     t.index ["tsv"], name: "index_spending_proposals_on_tsv", using: :gin
   end
 
+  create_table "stats_versions", force: :cascade do |t|
+    t.string   "process_type"
+    t.integer  "process_id"
+    t.datetime "created_at",   null: false
+    t.datetime "updated_at",   null: false
+    t.index ["process_type", "process_id"], name: "index_stats_versions_on_process_type_and_process_id", using: :btree
+  end
+
   create_table "taggings", force: :cascade do |t|
     t.integer  "tag_id"
     t.integer  "taggable_id"

spec/models/poll/stats_spec.rb

@@ -231,4 +231,30 @@ 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}"
+
+        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/stats_version_spec.rb

@@ -0,0 +1,13 @@
+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