https://blog.rebased.pl/2021/01/25/what-week-is-it-anyway-1.html

stats.rb

class Stats
  class << self
    def weekly_progress(issue_ids)
      relation = Issue.where(id: issue_ids)
                      .order('year asc, week asc')
                      .group('year, week')

      all = <<-SQL
        date_part('week', created_at) as week,
        date_part('year', created_at) as year,
        count(*) as total
      SQL
      
      all_sql = relation.select(all).to_sql
      weekly_progress_output(issues_report(all_sql))
    end

    private

    def issues_report(request)
      {}.tap do |hash|
        ActiveRecord::Base.connection.select_all(request).each do |row|
          week = [row['year'].to_i, row['week'].to_i].join('-')
          hash[week] = row['total'].to_i
        end
      end
    end

    def extract_date(data, type)
      week_parts = data.keys.first.split("-") if type == 'first'
      week_parts = data.keys.last.split("-") if type == 'last'
      year, week = week_parts[0].to_i, week_parts[1].to_i
      n = 0
      begin
        Date.commercial(year, week, 1)
      rescue ArgumentError
        raise if (n+=1) > 3
        year -= 1 if week > 50
        retry
      end
    end

    def weekly_progress_output(all)
      first_date = extract_date(all, 'first')
      last_date =  extract_date(all, 'last')

      (first_date..last_date).step(7).map do |date|
        week = [date.strftime("%G"), date.strftime("%V").to_i].join('-')
        [
          week,
          all[week] || 0
        ]
      end
    end
  end
end

stats_spec.rb

require 'spec_helper'

describe Stats do
  describe '#weekly_progress' do
    subject { Stats.weekly_progress(issue_ids) }

    let(:issue) do
      create(:issue).tap do |_issue|
        # Make spec pass on a sunday evening.
        _issue.update_column :created_at,
          (_issue.created_at.beginning_of_week + 1.day)
      end
    end

    let(:issue_ids)      { [issue.id] } 
    let(:stats)          { subject[0] }
    let(:formatted_date) { stats.first }
    let(:issue_count)    { stats.second }
    let(:created_at)     { issue.created_at }

    it 'returns correct weekly stats' do
      expect(subject.length).to eq(1)
      expect(issue_count).to eql(1)
      expect(formatted_date).to eql([created_at.strftime("%Y"), created_at.strftime("%V").to_i].join("-"))
    end
  end
end