Caius Theory

Now with even more cowbell…

#to_param and keyword slugs

Imagine you've got a blogging app and it's currently generating URL paths like posts/10 for individual posts. You decide the path should contain the post title (in some form) to make your URLs friendlier when someone reads them. I know I certainly prefer to read vs (That's a fun blog post if you're into (ab)using ruby occasionally!)

Now you know all about how to change the URL path that rails generates—just define to_param in your app. Something simple that generates a slug consisting of hyphens and lowercase alphanumerical characters. For example:

# 70-abusing-ruby-1-9-json-for-fun
def to_param
  "#{id}-#{title.gsub(/\W/, "-").squeeze("-")}".downcase

NB: You might want to go the route of storing the slug against the post record in the database and thus generating it before saving the record. In which case the rest of this post is sort of moot and you just need to search on that column. If not, then read on!

Now we're generating a nice human-readable URL we need to change the way we find the post in the controller's show action. Up until now it's been a simple @post = Post.find(params[:id]) to grab the record out the database. Problem now is params[:id] is "70-abusing-ruby-1-9-json-for-fun", rather than just "70". A quick check in the String#to_i docs reveals it "Returns the result of interpreting leading characters in str as an integer base base (between 2 and 36)." Basically it extracts the first number it comes across and returns it.

Knowing that we can just lean on it to extract the id before using find to look for the post: @post = Post.find(params[:id].to_i). Fantastic! We've got nice human readable paths on our blog posts and they can be found in the database. All finished… or are we?

There's still a rather embarassing bug in our code where we're not explicitly checking the slug in the URL against the slug of the Post we've extracted from the database. If we visited /posts/70-ruby-19-sucks-and-python-rules-4eva it would load the blog post and render it without batting an eyelid. This has caused rather a few embarrassing situations for some high profile media outlets who don't (or didn't) check their URLs and just output the content. Luckily there's a simple way for us to check this.

All we want to do is render the content if the id param matches the slug of the post exactly, and return a 404 page if it doesn't. We already know the id param (params[:id]) and have pulled the Post object out of the database and stored it in an instance variable (@post). The @post knows how to generate it's own slug, using #to_param.

So we end up with something like the following in our posts controller, which does all the above and correctly returns a 404 if someone enters an invalid slug (even if it starts with a valid post id):

def show
  @post = Post.find(params[:id].to_i)
  render_404 && return unless params[:id] == @post.to_param

def render_404
  render :file => Rails.root + "public/404.html", :status => :not_found

And going to an invalid path like /posts/70-ruby-19-sucks-and-python-rules-4eva just renders the default rails 404 page with a 404 HTTP status. (If you want the id to appear at the end of the path, alter to_param accordingly and do something like params[:id].match(/\d+$/) to extract the Post's id to search on.)

Hey presto, we've implemented human readable slugs that are tamper-proof (without storing them in the database.)

(And bonus points if in fact you spotted I used my blog as an example, but that it isn't a rails app. (Nor contains the blog post ID in the pretty URL.) It's actually powered by Habari at the time of posting!

Abusing Ruby 1.9 & JSON for fun

Ever since I found out about the new hash syntax you can use in ruby 1.9, and how similar that syntax is to JSON, I've been waiting for someone to realise you can just abuse eval() for parsing (some) JSON now.

For example, lets say we have the following ruby hash, which could be generated by a RESTful api:

thing = {
    :person => {
        :name => "caius"

If we pull in the JSON gem and dump that out as a string, we get the following:

jsonstr = thing.to_json
# => '{"person":{"name":"caius"}}'

That's… not quite what we wanted. It's not going to turn back into valid ruby as it is. Luckily javascript will parse objects without requiring the attributes to be wrapped in quotes, eg: {some: "attribute"}. We could build a JSON emitter that does it properly, or we could just run it through a regular expression instead. (Lets also add a space after the colon to aid readability.)

jsonstr.gsub!(/"([^"]+)": /, '\1: ')
# => '{person: {name: "caius"}}'

Okay, so now we've turned a ruby hash into a JSON hash, that can still be parsed by the browser. Here's a screenshot to prove that:

Valid JSON 'thing'

As you can see, it parses that into a valid JS object, complete with person and then (nested) name attributes. If we wanted to, thing["person"]["name"] or would access the nested value "caius" just fine.

Now then, we've proved that is successfully parsed into javascript objects by the browser, generated from a ruby hash. No great shakes there, that's fairly simple and has worked for ages. Now for my next trick, I'm going to turn that string of JSON back into a ruby hash, all without going anywhere near the JSON gem.

Some of you might have guessed what I'm about to do and have started hoping you've guessed wrongly — just for the record I don't condone doing this except for fun and games. The JSON gem is there for a reason ;) With that little disclaimer out the way, here we go!

thing2 = eval(jsonstr)
# => {:person=>{:name=>"caius"}}
thing2 == thing
# => true

Oh snap! We just turned javascript objects back into valid ruby objects, in one simple method call. And we'd be able to access the "caius" value by calling thing2[:person][:name], or creating OpenStructs from the hashes and calling Which is uncannily like the JS!

Updated 2011-02-07: Jens Ayton pointed out unquoted keys aren't strictly valid JSON, which is correct. Amended to say they're parsed as javascript objects instead, with no mention of it being valid JSON.

Ruby Shortcuts

There's a few useful shorthand ways to create certain objects in Ruby, a couple of obvious ones are [] to create an Array and {} to create a Hash (Or block/Proc). There's some not so obvious ones too, for creating strings, regexes and executing shell commands.

With all of the examples I've used {} as the delimiter characters, but you can use a variety of characters. Personally I tend to use {} unless the string contains them, in which case I'll use // or @@. My only exception appears to be %w, for which I tend to use ().


% and %Q are the same as using double quotes, including string interpolation. Really useful when you want to create a string that contains double quotes, but without the hassle of escaping them.

%{}                 # => ""
%Q{}                # => ""

%{caius}            # => "caius"
%{caius #{5}}       # => "caius 5"
%{some "foo" thing} # => "some \"foo\" thing"

%q is equivalent to using single quotes. Behaves exactly the same, no string interpolation.

%q{}           # => ''
%q{caius}      # => "caius"
%q{caius #{5}} # => "caius \#{5}"


%w is the equivalent of using String#split. It takes a string and splits it on whitespace. With the added bonus of being able to escape whitespace too. %W allows string interpolation.

%w(foo bar sed)  # => ["foo", "bar", "sed"]
%w(foo\ bar sed) # => ["foo bar", "sed"]
%W(foo #{5} bar) # => ["foo", "5", "bar"]


%r is just like using // to create a regexp object. Comes in handy when you're writing a regex containing / as you don't have to continually escape it.

%r{foo|bar} # => /foo|bar/
%r{foo/bar} # => /foo\/bar/


%s creates a symbol, just like writing :foo manually. It takes care of escaping the symbol, but unlike :"" it doesn't allow string interpolation however.

%s{foo}      # => :foo
%s{foo/bar}  # => :"foo/bar"
:"foo-#{5}"  # => :"foo-5"
%s{foo-#{5}} # => :"foo-\#{5}"

Shelling out

%x is the same as backticks (``), executes the command in a shell and returns the output as a string. And just like backticks it supports string interpolation.

`echo hi`     # => "hi\n"
%x{echo hi}   # => "hi\n"
%x{echo #{5}} # => "5\n"

Ignore .gitignore in Git

Recently I ran into an issue where I was working on a project which had files I wanted git to ignore, but I didn't want to commit a .gitignore file into the project. In case you don't know, any files matching a pattern in .gitignore in a git repository are ignored by git. (Unless the file(s) have already been committed, then they need removing from git before they are ignored.)

Initially I figured I could just throw the patterns I needed excluded into my global ~/.gitignore, but quickly realised that I needed files matching these patterns to show up in other git repos, so going the global route really wasn't an option. After some thought I wondered if you could make git ignore .gitignore, whilst still getting it to ignore files matching the other patterns in the .gitignore.

Lets create a new empty repo to test this crazy idea in:

$ mkdir foo
$ cd foo
$ git init
Initialized empty Git repository in /Volumes/Brutus/Users/caius/foo/.git/

And create a couple of files for us to play with:

$ touch bar
$ touch baz

Ignore one of the files so we can check other matches are still ignored later on:

$ echo "baz" >> .gitignore
$ git status
# On branch master
# Initial commit
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#       .gitignore
#       bar
nothing added to commit but untracked files present (use "git add" to track)

Ok so far, but we can still see .gitignore in git, so now for the crazy shindig, ignore the ignore file:

$ echo ".gitignore" >> .gitignore 

Lets see if it worked, or if we can still see our .gitignore:

$ git status
# On branch master
# Initial commit
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#       bar
nothing added to commit but untracked files present (use "git add" to track)

And lets just double-check that .gitignore and baz still exist on the filesystem:

$ ls -a
.  ..  .git  .gitignore  bar  baz

Fantastic! Turns out adding ".gitignore" to .gitignore works perfectly. The file is still parsed by git to ignore everything else too, so it does exactly what I needed in this instance.

Mac Tips you may not know

Here are some mac tips I know and consider "basic" mac knowledge, but no-one else seems to know.

(Alternative title Peter Cooper suggested, "A miscellany of input device co-ordinations to modulate the response of Apple's task preview and switching subsystem")