scinote-web/app/models/user_notification.rb

class UserNotification < ActiveRecord::Base
  belongs_to :user
  belongs_to :notification

  def self.recent_notifications(user)
    Notification.joins(:user_notifications)
                .where('user_notifications.user_id = ?', user.id)
                .order(created_at: :DESC)
                .limit(10)
  end

  def self.unseen_notification_count(user)
    where('user_id = ? AND checked = false', user.id).count
  end

  def self.seen_by_user(user)
    where(user: user).map do |n|
      n.checked = true
      n.save
    end
  end
end