expert.rb

class Expert < ActiveRecord::Base
  include Notifiable
  
  has_many :trackings do
    def recent
      where('created_at >= ?', proxy_association.owner.notified_at)
    end
  end
end

notifiable.rb

require 'active_support/concern'

module Notifiable
  extend ActiveSupport::Concern
  
  included do
    scope :notified,              -> { where.not(notified_at: nil) }
    scope :notified_one_week_ago, -> { notified.where('notified_at <= ?', 1.week.ago) }
    scope :with_recent_trackings, -> { joins(:trackings).where('trackings.created_at >= users.notified_at') }
  end
  
  class_methods do
    def deliver_weekly_notifications
      notified_one_week_ago.with_recent_trackings.each do |expert|
        expert.deliver_weekly_notification
      end
    end 
  end
  
  def notified?
    !!notified_at
  end
  
  def deliver_first_notification
    NotificationMailer.first_notification(self).deliver
  end
  
  def deliver_weekly_notification
    NotificationMailer.weekly_notification(self).deliver
  end
end

notification_mailer.rb

class NotificationMailer < ActionMailer::Base
  after_action :set_notified_at
  
  def weekly_notification(expert)
    @expert = user
    @trackings = @expert.trackings.recent
    
    mail to: @expert.email, subject: 'Weekly Notification'
  end
  
  def first_notification(expert)
    @expert = expert
    @tracking = @expert.trackings.first
    
    mail to: @expert.email, subject: 'First notification'
  end
  
private

  def set_notified_at
    @expert.update_column(:notified_at, Time.current)
  end
end

schedule.rb

every 15.minutes do
  Expert.deliver_weekly_notifications
end

tracking.rb

class Tracking < ActiveRecord::Base
  belongs_to :expert
  
  after_create :deliver_first_notification, unless: -> tracking { tracking.expert.notified? }
  
private

  def deliver_first_notification
    expert.deliver_first_notification
  end
end