gistfile1.rb

class Challenge
  def user_with_highest stat = :distance
    User.with_highest stat, period, users
  end

  def period
    start_date..end_date
  end
end

class User
  def self.with_highest stat, period = (10.years.ago)..(10.years.from_now), users = all
    users.max { |a, b| a.activities.total_stat_between(period, stat) > b.activities.total_stat_between(period, stat) }
  end
end

class Activity
  def self.total_stat_between(period, stat)
    between(period).sum(stat)
  end

  def self.between(period)
    where(date: period)
  end
end