Group people by age instead of birth year in stats

When calculating the stats on, say, January 1st 2024, and using a group
age of, say, between 20 and 24 years, we were considering that everyone
born between 2000 and 2004 had between 20 and 24 years. This wasn't
accurate, since most people born in 2004 would have 19 years at that
point, and most people born in 1999 would have 24 years.

app/models/user.rb

@@ -120,7 +120,10 @@ class User < ApplicationRecord
     where("username ILIKE ? OR email ILIKE ? OR document_number ILIKE ?", search, search, search)
   end
   scope :between_ages, ->(from, to, at_time: Time.current) do
-    where(date_of_birth: (at_time - to.years).beginning_of_year..(at_time - from.years).end_of_year)
+    start_date = at_time - (to + 1).years + 1.day
+    end_date = at_time - from.years
+
+    where(date_of_birth: start_date.beginning_of_day..end_date.end_of_day)
   end
 
   before_validation :clean_document_number

spec/models/budget/stats_spec.rb

@@ -159,7 +159,7 @@ describe Budget::Stats do
   describe "#participants_by_age" do
     it "returns the age groups hash" do
       [21, 22, 23, 23, 34, 42, 43, 44, 50, 51].each do |age|
-        create(:user, date_of_birth: budget.phases.balloting.ends_at - age.years)
+        create(:user, date_of_birth: budget.phases.balloting.ends_at - age.years - rand(0..11).months)
       end
 
       allow(stats).to receive(:participants).and_return(User.all)
@@ -182,7 +182,7 @@ describe Budget::Stats do
       budget = travel_to(50.years.ago) { create(:budget, :finished) }
 
       [21, 22, 23, 23, 34, 42, 43, 44, 50, 51].each do |age|
-        create(:user, date_of_birth: budget.phases.balloting.ends_at - age.years)
+        create(:user, date_of_birth: budget.phases.balloting.ends_at - age.years - rand(0..11).months)
       end
 
       stats = Budget::Stats.new(budget)

spec/models/poll/stats_spec.rb

@@ -169,7 +169,7 @@ describe Poll::Stats do
     it "returns stats based on what happened when the voting took place" do
       travel_to(100.years.ago) do
         [16, 18, 32, 32, 33, 34, 64, 65, 71, 73, 90, 99, 105].each do |age|
-          create(:user, date_of_birth: age.years.ago)
+          create(:user, date_of_birth: age.years.ago - rand(0..11).months)
         end
 
         create(:poll, starts_at: 1.minute.from_now, ends_at: 2.minutes.from_now)

spec/models/user_spec.rb

@@ -438,6 +438,80 @@ describe User do
         expect(User.between_ages(51, 100).count).to eq 1
       end
 
+      it "does not consider that people born in the same year share the same age" do
+        april_1st_user = create(:user, date_of_birth: Time.zone.local(2001, 4, 1))
+        april_2nd_user = create(:user, date_of_birth: Time.zone.local(2001, 4, 2))
+        april_3rd_user = create(:user, date_of_birth: Time.zone.local(2001, 4, 3))
+
+        travel_to "2021-04-02" do
+          expect(User.between_ages(18, 19)).to eq [april_3rd_user]
+          expect(User.between_ages(20, 21)).to match_array [april_1st_user, april_2nd_user]
+          expect(User.between_ages(19, 20)).to match_array [april_1st_user, april_2nd_user, april_3rd_user]
+        end
+      end
+
+      it "works on leap years with users born on leap years" do
+        leap_day_user = create(:user, date_of_birth: Time.zone.local(2000, 2, 29))
+        march_1st_user = create(:user, date_of_birth: Time.zone.local(2000, 3, 1))
+
+        travel_to "2024-02-28" do
+          expect(User.between_ages(22, 23)).to match_array [leap_day_user, march_1st_user]
+          expect(User.between_ages(23, 24)).to match_array [leap_day_user, march_1st_user]
+          expect(User.between_ages(24, 25)).to eq []
+        end
+
+        travel_to "2024-02-29" do
+          expect(User.between_ages(22, 23)).to eq [march_1st_user]
+          expect(User.between_ages(23, 24)).to eq [leap_day_user, march_1st_user]
+          expect(User.between_ages(24, 25)).to eq [leap_day_user]
+        end
+
+        travel_to "2024-03-01" do
+          expect(User.between_ages(22, 23)).to eq []
+          expect(User.between_ages(23, 24)).to match_array [leap_day_user, march_1st_user]
+          expect(User.between_ages(24, 25)).to match_array [leap_day_user, march_1st_user]
+        end
+      end
+
+      it "works on non-leap years with users born on leap years" do
+        leap_day_user = create(:user, date_of_birth: Time.zone.local(2000, 2, 29))
+        march_1st_user = create(:user, date_of_birth: Time.zone.local(2000, 3, 1))
+
+        travel_to "2023-02-28" do
+          expect(User.between_ages(21, 22)).to match_array [leap_day_user, march_1st_user]
+          expect(User.between_ages(22, 23)).to match_array [leap_day_user, march_1st_user]
+          expect(User.between_ages(23, 24)).to eq []
+        end
+
+        travel_to "2023-03-01" do
+          expect(User.between_ages(21, 22)).to eq []
+          expect(User.between_ages(22, 23)).to match_array [leap_day_user, march_1st_user]
+          expect(User.between_ages(23, 24)).to match_array [leap_day_user, march_1st_user]
+        end
+      end
+
+      it "works on leap years with users born on non-leap years" do
+        march_1st_user = create(:user, date_of_birth: Time.zone.local(2001, 3, 1))
+
+        travel_to "2024-02-28" do
+          expect(User.between_ages(21, 22)).to eq [march_1st_user]
+          expect(User.between_ages(22, 23)).to eq [march_1st_user]
+          expect(User.between_ages(23, 24)).to eq []
+        end
+
+        travel_to "2024-02-29" do
+          expect(User.between_ages(21, 22)).to eq [march_1st_user]
+          expect(User.between_ages(22, 23)).to eq [march_1st_user]
+          expect(User.between_ages(23, 24)).to eq []
+        end
+
+        travel_to "2024-03-01" do
+          expect(User.between_ages(21, 22)).to eq []
+          expect(User.between_ages(22, 23)).to match_array [march_1st_user]
+          expect(User.between_ages(23, 24)).to match_array [march_1st_user]
+        end
+      end
+
       it "returns users between certain ages on a reference date" do
         reference_date = 20.years.ago