Some Random Blog

On my latest project, I discovered I had to pre-populate the project's database with existing content. Jon Udell just posted about how much of a waste of time this can be in some circumstances, but in this case, Hpricot and database migrations made it easy. This wouldn't be a solution I'd use if I needed the data as anything beyond a one-off bootstrap, but in this case it worked really well.

Hpricot, for those who don't know, is an HTML parser for Ruby that's fun to use. When I was first learning Ruby, most of the simplest yet useful projects I could come up with used Hpricot to grab content off of websites and format or combine it in different ways. Its syntax looks like this:

require 'hpricot'
require 'open-uri'

uri = URI.parse(link)
doc = Hpricot(open(uri))

name = (doc/"li.active a").inner_html
page_title = (doc/"title").inner_html
body = (doc/"#content_body").html

In this example, Hpricot is using CSS selectors to grab different pieces of content out of the page in link. The nice thing about using CSS selectors here is the code tends to be less fragile than screenscrapers that depend on the architecture of the page.

Page scraping can be a frustrating art, especially if the page layout changes or if pages are inconsistent, or have unique properties. Luckily, in this case, I only had to get it right once, and even then, I didn't have to get it completely right. I used this four-stage process:

1. Use Hpricot to get as much data off the page and into our data structures as possible.
2. Persist this data to the database, and make appropriate changes that Hpricot missed, or couldn't catch.
3. Dump the database to a file, and use it to bootstrap our production database.
4. Repeat until finished.

Rails database migrations made this relatively easy. I ended up with three migrations. The first migration created the structure of the database. The second loaded the current page data dump from the dump file. The third grabbed a few pages I still needed to parse, and I was left with data that I could tweak and dump, overwriting it with a dump containing all the page data (including the stuff I just tweaked). I could then blow away the database and repeat until I didn't have any more pages to parse.

This worked perfectly, since I didn't have to spend time getting my Hpricot parsing perfect (since I could modify the resulting data using our CMS and re-dump), and I was left with a dump of all the data that I needed in order to dynamically generate these formerly mostly static pages.

This article made it across my RSS reader today. I ran into my own problem with this while writing a custom CMS for work. We wanted to have reusable components that could be added to CMS pages, which could take various parameters, could be cached, and could be viewed in different ways given a size. I investigated Rails components at work, but noticed that using those is discouraged by the Rails community.

My investigation brought me to Typo’s sidebar model, which I used as the basis for the model we ended up using for the prototype of the project. The ultra-simplified version of the model works like this:

We have a Sidebar base class, which inherits from ActiveRecord::Base. Sidebars inherit from this Sidebar class.

Which gives us something like this:

class Sidebar < ActiveRecord::Base
  serialize :config

  class << self

    def params
       @params ||= []
    end

    def param(name, type, options = {})
      params << options.merge({:name => name, :type => type})
      self.send(:define_method, name) do 
        self.config[name] || options[:default]
      end
      self.send(:define_method, "#{name}=") do |value|
        self.config[name] = value
      end
    end
  end
end

class StaticTextSidebar < Sidebar
  param :content, :text, :default => "Hello, World!"
end

So now we have a way of defining sidebars and their parameters. The metaprogramming in the Sidebar base class allows us to programatically query the parameters declared in a Sidebar. This will be important later. For now, we still need to declare the view of a sidebar, so we do it in _static_text_sidebar.rhtml:

<%= sidebar.content %>

Now, we add a helper to application_helper.rb to render the sidebar:

def render_sidebar(sidebar)
  render :partial => sidebar.class.name.underscore, :locals => { :sidebar => sidebar }
end

and then we can call render_sidebar in any of our views on an instance of a sidebar to render it. It’s not perfect, but it’s good enough for a prototype!

From here, we have a very basic reusable model-view framework that we can include in any of our pages. Sidebar instances can be associated with content on a page to be displayed, and their configuration can be serialized to the database along with the items they display with.

Creating and configuring sidebars can be done programmatically, by generating a form based on the parameters a sidebar takes and placing that form data into the sidebar, the same way one would with a standard ActiveRecord object. Their parameters can be validated using standard Rails validations and the result of the render_sidebar call can be cached.

This basic idea, with a little bit of work, can easily form the basis for a simple reusable component architecture, and we’ve been having a ton of success with it so far.

I love programming languages. Learning a new one is one of my favorite computer-related activities to do. Right now my favorite is Ruby for reasons I'll get into at another time. There are a few others, though, that have caught my attention lately:

• Erlang: There's something really powerful about a language where remote procedure calls are built into the language. I also realized (again) how cool tail recursion can be. I don't have any ideas trying to get out of my head that would find Erlang useful now, but I really want to play with it more.
• Scala: Scala is an interesting language. It's strongly typed with type inference, which I find very cool and wish more languages would try. It also supports the actor model (like Erlang) and some really crazy generic programming constructs. To me it seems like magic. It also runs on the JVM, which means I get every library people can think of right off the bat. Score!

I also found the article by Stostroup here to be really interesting. It's about 50 pages long, but well worth reading if you're interested in or (are forced to) use C++ at all.