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Simple caching of markdown markdown in your model


As I’ve written about it before, I’m using markdown for this blog. Originally, I would do the translation from markdown to html every time a page is rendered. Seems pretty inefficient, right? Yeah, not so much.

As it turns out, it is really simple to cache this information.

For a little background, here’s the schema for my posts:

create_table "posts", :force => true do |t|
  t.string   "title"
  t.text     "content"
end

And the model:

class Post < ActiveRecord::Base
  validates_presence_of :title
  validates_presence_of :content
end

What I want to do is, before a post is saved, to generate html from the markdown and save it to the database. Then, instead of generating it at page render, use the saved html.

ActiveRecord provides several callbacks into its lifecycle. There are a few ways you can add a callback as the RDoc demonstrates. The lifecycle we’re really interested is before_save. This is for whenever the post is saved, regardless of it is creating a new one, or updating an existing one.

Before we implement the callback, let’s update our schema with a migration.

$ script/generate migration AddCachedContent
class AddCachedContent < ActiveRecord::Migration
  def self.up
    add_column :posts, :cached_content, :text
  end

  def self.down
    remove_column :posts, :cached_content
  end
end

Hmm… one problem: we probably want to cache the content of all existing posts. So, let’s add a method that will cache the content.

class Post < ActiveRecord::Base
  validates_presence_of :title
  validates_presence_of :content
  
  def cache_content
    self.cached_content = BlueCloth.new(self.content).to_html
  end
end

Now we have the means to cache our markdown, we can update our migration to cache the content of each post and save it:

class AddCachedContent < ActiveRecord::Migration
  def self.up
    add_column :posts, :cached_content, :text

    Post.find(:all).each do |post|
      post.cache_content
      post.save!
    end
  end

  def self.down
    remove_column :posts, :cached_content
  end
end

We have everything in place to go ahead and actually add the callback:

class Post < ActiveRecord::Base
  validates_presence_of :title
  validates_presence_of :content
  
  before_save :cache_content
  def cache_content
    self.cached_content = BlueCloth.new(self.content).to_html
  end
end

We’re just about done. All that remains is to update places in the view that generate html from markdown to just use the cached_content instead.

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