statistics_code.rb

# app/model/weekly_day.rb
WeeklyDay = Struct.new(:start_day, :end_day) do
  def start_day_string
    start_day.strftime('%A %d-%m-%Y')
  end
end

# app/model/daily_statistic.rb
DailyStatistic = Struct.new(:day_string, :orders) do
  def day
    day_string
  end

  def orders_sum
    @orders_sum ||= orders.map(&:price).reduce(:+)
  end

  def pizza_count
    @pizza_count ||=  orders
                      .map { |order| order.items.where(type: :pizza).count }
                      .reduce(:+)
  end

  def users
    @users ||= get_users_arr
  end

  def pizza
    @pizza ||= get_pizza_arr
  end

  private

  ##
  # returns array
  # like this
  # [ { pizza: <Item>, count: 10},
  #   { pizza: <Item>, count: 8},
  #   { pizza: <Item>, count: 7},]

  def get_pizza_arr
    pizza_hash = Hash.new(0)
    orders
      .map { |order| order.items.where(type: :pizza) }
      .reduce(:+).each { |item| pizza_hash[item.id] += 1 }
    pizza_hash = pizza_hash.sort_by { |k, v| v }.reverse.to_h
    Hash[pizza_hash.sort_by { |k,v| -v }[0..2]].map do |k,v|
      {
        pizza: Item.find(k),
        count: v
      }
    end
  end

  ##
  # return array like this
  # [ {user: <User>, sum: 1000},
  #   {user: <User>, sum: 900},
  #   {user: <User>, sum: 850}]

  def get_users_arr
    users_hash = Hash.new(0)
    orders.each { |order| users_hash[order.user_id] += order.price }
    users_hash = users_hash.sort_by { |k, v| v }.reverse.to_h
    Hash[users_hash.sort_by { |k,v| -v }[0..2]].map do |k,v|
      {
        user: User.find(k),
        sum: v
      }
    end
  end
end

# app/models/weekly_statistic.rb
class WeeklyStatistic
  def perform
    date.map do |weekly_day|
      DailyStatistic.new(
        weekly_day.start_day_string,
        get_orders(weekly_day)
      )
    end
  end

  def date
    @date ||= get_date
  end

  private

  ##
  # it returns something like this
  # [ <WeeklyDay>, <WeeklyDay>, <WeeklyDay> ]

  def get_date
    arr = []
    start_day = nil
    loop do
      unless start_day.present?
        start_day = DateTime.current.beginning_of_week
      else
        start_day += 1.day
      end
      end_day = start_day + 1.day

      arr.push(WeeklyDay.new(start_day, end_day))
      break if end_day > DateTime.current.end_of_week
    end
    arr
  end

  def get_orders(date_obj)
    Order.preload(:user, :items).where(
      'created_at >= :start_day AND created_at < :end_day',
      start_day: date_obj.start_day,
      end_day: date_obj.end_day
    )
  end
end

# app/controllers/statistics_controller.rb
class StatisticsController < ApplicationController
  # ...
  def weekly
    @weekly = WeeklyStatistic.new.perform
  end
  # ...
end