poro-service-generate-weekly-report.rb

module Report
  class GenerateWeekly
    WeeklyReport = Struct.new(
      :week_number,
      :date_from,
      :date_to,
      :count_entries,
      :avg_distance,
      :avg_speed
    )

    def initialize(user)
      @user = user
    end

    def call
      @user.entries.group_by(&:week).map do |week, entries|
        WeeklyReport.new(
         week,
         weeks_to_date_from(week),
         weeks_to_date_to(week),
         entries.count,
         avg_distance(entries),
         avg_speed(entries)
        )
      end
    end

    private

    def weeks_to_date_from(week)
      (Date.new + week.to_i.weeks).to_s.split(',')[0]
    end

    def weeks_to_date_to(week)
      (Date.new + week.to_i.weeks + 7.days).to_s.split(',')[0]
    end

    def avg_distance(entries)
      distances = entries.inject(0){|sum, n| sum + n.distance }
      (distances / entries.count).round(2)
    end

    def avg_speed(entries)
      speeds = entries.inject(0){|sum, n| sum + n.speed }
      (speeds / entries.count).round(2)
    end
  end
end

Since user is referred to once to get the list of entries, why not pass in only what's necessary (in this case, user.entries) as an argument to #initialize?

While there is no absolute true, I find passing the current user more inclined to SRP. This way, your service has the responsibility of building the report to the passed user and not only parse the data. This has it's advantages since the controller does not need to know which data the service will use to build the report.