Generate stats on every request without caching them

In commit e51e034, we started using the same code to show stats in the
public area and in the admin area. However, in doing so we introduced a
bug, since stats in the public area are only shown after a certain part
of the process has finished, meaning the stats appearing on the page
never change (in theory), so it's perfectly fine to cache them. However,
in the admin area stats can be accessed while the process is still
ongoing, so caching the stats will lead to the wrong results being
displayed.

We've thought about expiring the cache when new supports or ballot lines
are added; however, that means the methods calculating the stats for the
supporting phase would expire when supports are added/removed but the
methods calculating the stats for the voting phase would expire when
ballot lines are added/removed. It gets even more complex because the
`headings` method calculates stats for both the supporting and the
voting phases.

But, since now we're using a temporary table to calculate the stats,
generating them might take between 1 and 2 seconds even in processes
with a hundred thousand participants, meaning we can disable the cache
and generate the stats on every request instead. It might not scale so
well when there are millions of participants; we might re-enable the
cache or find a different way to optimize the stats calculations if we
ever get to that point.

Note that, since we're only using the temporary table inside the
`generate` method, we need to cache the results obtained inside this
method so they're available elsewhere. We're using instance variables
for this purpose. This means the code doesn't improve much with this
change, but at least we don't have to worry about when we should expire
the cache.

Since loading stats in the admin section is fast even without the cache
because they only load very basic statistics, we're not even generating
them first in this case, so everything works the same way it did before
commit e51e034.

Co-authored-by: Javi Martín <javim@elretirao.net>

app/models/budget/stats.rb

@@ -179,8 +179,4 @@ def supports(supportable)
     end
 
     stats_cache(*stats_methods)
-
-    def full_cache_key_for(key)
-      "budgets_stats/#{budget.id}/#{phases.join}/#{key}"
-    end
 end

app/models/concerns/statisticable.rb

@@ -222,7 +222,13 @@ def range_description(start, finish)
       end
     end
 
-    def stats_cache(key, &)
-      Rails.cache.fetch(full_cache_key_for(key), expires_in: 1.hour, &)
+    def stats_cache(key, &block)
+      instance_variable_name = key.to_s.gsub("?", "_question")
+
+      unless instance_variable_get("@#{instance_variable_name}")
+        instance_variable_set("@#{instance_variable_name}", block.call)
+      end
+
+      instance_variable_get("@#{instance_variable_name}")
     end
 end

app/models/poll/stats.rb

@@ -116,8 +116,4 @@ def total_unregistered_booth
     end
 
     stats_cache(*stats_methods)
-
-    def full_cache_key_for(key)
-      "polls_stats/#{poll.id}/#{key}"
-    end
 end

spec/system/admin/stats_spec.rb

@@ -111,6 +111,33 @@
         expect(page).to have_link "Go back", count: 1
       end
 
+      scenario "Clear cache after new supports", :with_cache do
+        budget.update!(phase: :selecting)
+        investment = create(:budget_investment, heading: heading_all_city)
+        create(:user, :level_two, votables: [investment])
+
+        supporter = create(:user, :level_two)
+
+        visit budget_supporting_admin_stats_path(budget_id: budget)
+
+        expect(page).to have_content "VOTES\n1"
+        expect(page).to have_content "PARTICIPANTS\n1"
+
+        in_browser(:supporter) do
+          login_as(supporter)
+
+          visit budget_investment_path(budget, investment)
+          click_button "Support"
+
+          expect(page).to have_button "Remove your support"
+        end
+
+        refresh
+
+        expect(page).to have_content "VOTES\n2"
+        expect(page).to have_content "PARTICIPANTS\n2"
+      end
+
       scenario "hide final voting link" do
         visit admin_stats_path
         click_link "Participatory Budgets"
@@ -147,6 +174,32 @@
         expect(page).to have_content "VOTES\n3"
         expect(page).to have_content "PARTICIPANTS\n2"
       end
+
+      scenario "Clear cache after new votes", :with_cache do
+        budget.update!(phase: :balloting)
+        create(:user, ballot_lines: [investment])
+
+        balloter = create(:user, :level_two)
+
+        visit budget_balloting_admin_stats_path(budget_id: budget.id)
+
+        expect(page).to have_content "VOTES\n1"
+        expect(page).to have_content "PARTICIPANTS\n1"
+
+        in_browser(:balloter) do
+          login_as(balloter)
+
+          visit budget_investment_path(budget, investment)
+          click_button "Vote"
+
+          expect(page).to have_button "Remove vote"
+        end
+
+        refresh
+
+        expect(page).to have_content "VOTES\n2"
+        expect(page).to have_content "PARTICIPANTS\n2"
+      end
     end
   end