scope :filter_on_start_date, lambda { |date_start = nil, date_end = nil|
    if date_start.present? && date_end.present?
      where('start_at >= :start and start_at <= :end', start: date_start, end: date_end)
    elsif date_start.present?
      where('start_at >= :start', start: date_start)
    elsif date_end.present?
      where('start_at <= :end', end: date_end)
    else
      all
    end
  }