discourse/app/models/category.rb

class Category < ActiveRecord::Base

  belongs_to :topic
  belongs_to :user

  has_many :topics
  has_many :category_featured_topics
  has_many :featured_topics, through: :category_featured_topics, source: :topic

  has_many :category_featured_users
  has_many :featured_users, through: :category_featured_users, source: :user

  validates_presence_of :name
  validates_uniqueness_of :name
  validate :uncategorized_validator

  after_save :invalidate_site_cache
  after_destroy :invalidate_site_cache


  def uncategorized_validator
    return errors.add(:name, I18n.t(:is_reserved)) if name == SiteSetting.uncategorized_name
    return errors.add(:slug, I18n.t(:is_reserved)) if slug == SiteSetting.uncategorized_name
  end

  def self.popular
    order('topic_count desc')
  end

  def self.update_stats
    exec_sql "UPDATE categories
              SET topics_week = (SELECT COUNT(*)
                                FROM topics as ft
                                WHERE ft.category_id = categories.id
                                  AND ft.created_at > (CURRENT_TIMESTAMP - INTERVAL '1 WEEK')
                                  AND ft.visible),
                  topics_month = (SELECT COUNT(*)
                                FROM topics as ft
                                WHERE ft.category_id = categories.id
                                  AND ft.created_at > (CURRENT_TIMESTAMP - INTERVAL '1 MONTH')
                                  AND ft.visible),
                  topics_year = (SELECT COUNT(*)
                                FROM topics as ft
                                WHERE ft.category_id = categories.id
                                  AND ft.created_at > (CURRENT_TIMESTAMP - INTERVAL '1 YEAR')
                                  AND ft.visible)"
  end

  # Use the first paragraph of the topic's first post as the excerpt
  def excerpt
    if topic.present?
      first_post = topic.posts.order(:post_number).first
      body = first_post.cooked
      matches = body.scan(/\<p\>(.*)\<\/p\>/)
      if matches and matches[0] and matches[0][0]
        return matches[0][0]
      end
    end
    nil
  end

  def topic_url
    topic.try(:relative_url)
  end

  before_save do
    self.slug = Slug.for(self.name)
  end

  after_create do
    topic = Topic.create!(title: I18n.t("category.topic_prefix", category: name), user: user, visible: false)
    topic.posts.create!(raw: SiteSetting.category_post_template, user: user)
    update_column(:topic_id, topic.id)
    topic.update_column(:category_id, self.id)
  end

  # We cache the categories in the site json, so we need to invalidate it when they change
  def invalidate_site_cache
    Site.invalidate_cache
  end

  before_destroy do
    topic.destroy
  end

end