discourse/app/models/category.rb

class Category < ActiveRecord::Base
  belongs_to :topic, dependent: :destroy
  belongs_to :topic_only_relative_url, 
    select: "id, title",
    class_name: "Topic",
    foreign_key: "topic_id"
  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 :user_id
  validates_presence_of :name
  validates_uniqueness_of :name
  validate :uncategorized_validator

  after_save :invalidate_site_cache
  after_destroy :invalidate_site_cache

  scope :popular, lambda { order('topic_count desc') }

  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

  # Recalculates `topics_year`, `topics_month`, and `topics_week`
  # for each Category.
  def self.update_stats
    topics = Topic
               .select("COUNT(*)")
               .where("topics.category_id = categories.id")
               .where("categories.topic_id <> topics.id")
               .visible

    topics_year = topics.created_since(1.year.ago).to_sql
    topics_month = topics.created_since(1.month.ago).to_sql
    topics_week = topics.created_since(1.week.ago).to_sql

    Category.update_all("topics_year = (#{topics_year}),
                         topics_month = (#{topics_month}),
                         topics_week = (#{topics_week})")
  end

  def topic_url
    topic_only_relative_url.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, pinned_at: Time.now)

    post_contents = I18n.t("category.post_template", replace_paragraph: I18n.t("category.replace_paragraph"))
    topic.posts.create!(raw: post_contents, user: user)
    update_column(:topic_id, topic.id)
    topic.update_column(:category_id, self.id)
  end

  def self.post_template
    I18n.t("category.post_template", replace_paragraph: I18n.t("category.replace_paragraph"))
  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
end