Recent good notes

RSS feed
February 12, 2009
4 thanks

Real life use

If you’re wondering what the base64 format is used for, here are some examples:

  • HTTP Basic authentication: encode your username and password as one string, and add it as a header of an HTTP request. When a page requiring basic authentication gets called from a browser it results in a generic Username/Password dialog from that browser. See also http://en.wikipedia.org/wiki/Basic_access_authentication

  • Encode the binary content of images to base64 and embed it in XML documents, for example in web services

  • For more information see http://en.wikipedia.org/wiki/Base64

Just note that the encoded (character) data is about 30% larger than un-encoded (binary) data.

February 12, 2009
4 thanks

Binary files

Another real important flag is b when dealing with binary files. For example to download an mp3 from the internet you need to pass the b flag or the data will be screwed up:

# Downloads a binary file from the internet
require 'open-uri'
url = "http://fubar/song.mp3"
open(url, 'rb') do |mp3|
  File.open("local.mp3", 'wb') do |file|

Don’t say you haven’t been warned. :)

February 12, 2009
3 thanks

Other regular-expression modifiers

Likewise you can set Regexp::IGNORECASE directly on the regexp with the literal syntax:

# This will match "first", "First" and even "fiRSt"

Even more modifiers

  • o – Perform #{} interpolations only once, the first time the regexp literal is evaluated.

  • x – Ignores whitespace and allows comments in * regular expressions

  • u, e, s, n – Interpret the regexp as Unicode (UTF-8), EUC, SJIS, or ASCII. If none of these modifiers is specified, the regular expression is assumed to use the source encoding.

Literal to the rescue

Like string literals delimited with %Q, Ruby allows you to begin your regular expressions with %r followed by a delimiter of your choice.

This is useful when the pattern you are describing contains a lot of forward slash characters that you don’t want to escape:

# This will match "http://"
February 12, 2009
4 thanks

Literal syntax

As you propably know you can create an Array either with the constructor or the literal syntax:

Array.new == []
# => true

But there is also another nice and concise literal syntax for creating Arrays of Strings:

["one", "two", "three"] == %w[one two three]
# => true

You can use any kind of parenthesis you like after the %w, either (), [] or {}. I prefer the square brackets because it looks more like an array.

February 12, 2009
3 thanks

Useful scenario

This can be quite useful, for example when writing a command line script which takes a number of options.


Let’s say you want to make a script that can make the basic CRUD operations. So want to be able to call it like this from the command line:

> my_script create
> my_script delete

The following script allows you to use any abbreviated command as long as it is unambiguous.

# my_script.rb
require 'abbrev'

command = ARGV.first
actions = %w[create read update delete]
mappings = Abbrev::abbrev(actions)
puts mappings[command]

That means you can call it like this:

> my_script cr
> my_script d

And it will print:

February 10, 2009
8 thanks

Security issue with non-HTML formats

Please note that using default to_xml or to_json methods can lead to security holes, as these method expose all attributes of your model by default, including salt, crypted_password, permissions, status or whatever you might have.

You might want to override these methods in your models, e.g.:

def to_xml
  super( :only => [ :login, :first_name, :last_name ] )

Or consider not using responds_to at all, if you only want to provide HTML.

February 10, 2009
3 thanks

Cheat Sheet

I have written a short introduction and a colorful cheat sheet for Perl Compatible Regular Expressions (PCRE) as used by Ruby’s Regexp class:


February 9, 2009
5 thanks
February 7, 2009 - (>= v2.2.1)
3 thanks


This method is deprecated. You should use:

February 4, 2009
3 thanks

Multiline regexps

A shortcut for multiline regular expressions is

/First line.*Other line/m

(notice the trailing /m)

For example:

text = <<-END
  Hello world!
  This is a test.

text.match(/world.*test/m).nil?  #=> false
text.match(/world.*test/).nil?   #=> true
February 3, 2009
3 thanks

Possible gotcha

Please note that exists? doesn’t hold all the conventions of find, i.e. you can’t do:

Person.exists?(:conditions => ['name LIKE ?', "%#{query}%"]) # DOESN'T WORK!
February 3, 2009
3 thanks

Nothing here

You’re probably looking for I18n::Backend::Simple.

February 3, 2009
4 thanks

This method will still rewrite all the values of the table

Even if you update only a small boolean flag on your record, update_attribute will generate an UPDATE statement that will include all the fields of the record, including huge BLOB and TEXT columns. Take this in account.

February 1, 2009 - (v2.2.1)
3 thanks

You can't use Symbols, but you can use Regexps

You can’t use Symbol (although Symbol is accepted with render :action => :new), like:

assert_template :new # WON'T WORK!

But you can use Regexp, e.g.:

assert_template /new/ # WORKS OK

Note that the String matched with your Regexp is the full path to the template relative to the view/ directory of your app, so this will not work:

assert_template /^new$/ # WON'T WORK!

However this might:

assert_template /^employees\/new.html.haml$/
January 30, 2009 - (v1.2.0 - v2.1.0)
3 thanks

Hash conditions require explicit key and value

When condition passed as hash, the behavior is different from a finder method. Finder methods, such as:

find(:all, :user=>user)

will apply the user_id = user.id convention, provided user is an association (e.g. belongs_to :user). The exists? method will not do the same. You must specify the foreign key name and value explicitly, i.e:

January 27, 2009 - (>= v2.2.1)
5 thanks

Getting the object in a partial

If you need to get the object for the form inside a partial, and can’t use the instance variable, use the #object method… This is particularly useful when you’re dealing with single-table inheritance subclasses (e.g. MyOtherClass inherits from MyClass) or when you are using the same partial across different controllers.


<% form_for(@my_object) do %>
  <%= render :partial => 'form' %>
  <%= submit_tag 'Create' %>
<% end %>


<% if f.object.class.is_a? MyClass %>
 <%# do something... %>
<% elsif f.object.is_a? MyOtherClass %>
  <%# do something else... %>
<% end %>
January 27, 2009
6 thanks

Use collect in nested content_tags

Remember to use #collect instead of #each in nested content_tags

arr = ['a','b','c']
content_tag :div do 
  arr.collect { |letter| content_tag(:scan, letter) 
#=> <div>
#      <scan>a</scan>
#      <scan>b</scan>
#      <scan>c</scan>
#   </div>

If you used #each you would get this (which is probably a mistake):

#=> <div>
#      abc
#   </div>
January 26, 2009
5 thanks

two ways to disable single table inheritance

  1. Don’t use the column name ‘type’

  2. Or if the first is no option for you: Tell Rails to look for a not existing column like:

class MyModel < ActiveRecord::Base

  # disable STI
  inheritance_column = :_type_disabled
January 26, 2009 - (>= v2.2.1)
3 thanks

alternative (working with 2.2.X)


January 25, 2009
3 thanks

Routes = RouteSet.new

In config/routes.rb you can see this:

ActionController::Routing::Routes.draw do |map|

If you want to look at the code in ActionController::Routing you won’t find the definition of Routes. That’s because it’s actually an instance of the class RouteSet, defined in action_controller/routing.rb

Routes = RouteSet.new
January 24, 2009
8 thanks

How to test different responses of respond_to

You can shorten this:

@request.env['HTTP_ACCEPT'] = "application/rss"

To this:

@request.accept = "application/rss"

Also, if you send more than one Mime type, it will render the first one:

@request.accept = "text/javascript, text/html" #=> renders JS
@request.accept = "text/html, text/javascript" #=> renders HTML
January 22, 2009
7 thanks

Using fields_for with has_many associations

If you want to edit each element of an array of objects (such as with a has_many type association), you will need to include “[]” in your field parameter name, like so:

<% fields_for "object[]" do |subfield| -%>
 <% end -%>

Because you named the field parameter “object[]”, fields_for will assume you have an instance variable @object to use for the fields’ values. To fake this, you can do something like:

<% objects.each do |@object| -%>
  <% fields_for "object[]" do |subfield| -%>
  <% end -%>
<% end -%>

If that looks like sacrilegious Rails code to you, then you could consider:

<% objects.each do |object| -%>
  <% fields_for "object[]", object do |subfield| -%>
  <% end -%>
<% end -%>

In either case, params[:object] will be a hash where the ID of each object (determined via ActiveRecord::Base#to_param ) is associated with a hash of its new values:

params = { 'object' => { '123' => { 'field' => 'newval' }, '159' => { 'field' => 'newval' } } }
January 20, 2009 - (>= v2.2.1)
5 thanks

Reloading memoized values

Memoize is used to cache the result of a method. It’s roughly equivalent of having:

def memoized_method(*args)
  @result[args] ||= (
    # do calculation here

However, the result is cached so that it’s not calculated for every request.

To recalculate cached value use either



January 20, 2009
5 thanks

Javascript encoding DOES work!

grosser assertion is false :

mail_to('xxx@xxx.com', nil, :encode => :javascript)
# => "<script type=\"text/javascript\">eval(unescape('%64%6f%63%75%6d%65%6e%74%2e%77%72%69%74%65%28%27%3c%61%20%68%72%65%66%3d%22%6d%61%69%6c%74%6f%3a%78%78%78%40%78%78%78%2e%63%6f%6d%22%3e%78%78%78%40%78%78%78%2e%63%6f%6d%3c%2f%61%3e%27%29%3b'))</script>"

Use “nil” as the second parameter to tell mail_to that you want to use the first parameter for both text and email link

January 16, 2009 - (>= v2.2.1)
4 thanks

Remember to mixin the ActiveSupport::Memoizable module

To use memoize in your model you need to extend the model class with the module, like this:

class Person < ActiveRecord::Base
  # Mixin the module
  extend ActiveSupport::Memoizable

  def expensive_method
    # do something that is worth remembering
  memoize :expensive_method

If you use memoizable in most of your models you could consider mixing the module into all ActiveRecord models by doing this in an initializer:

January 16, 2009
7 thanks


Code example

# => true
January 15, 2009
4 thanks

Convert String to Class in Rails

ncancelliere gave us a very useful tip below, and I just want to make an addendum for it:

If you are using Rails, there is a CoreExtension for this called String#constantize.

"Foo::BarKeeper".constantize #=> Foo::BarKeeper

You can use it with String#camelize if you have to convert the name too

"foo/bar_keeper".camelize             #=> "Foo::BarKeeper"
"foo/bar_keeper".camelize.constantize #=> Foo::BarKeeper

Don’t forget to rescue NameError in case there was an invalid class name. :-)

January 14, 2009
6 thanks

File open permissions

Usage: File.open path, flags, [permissions]

Flags (bitmasks)







Read and write

If the file exists:







If the file doesn’t exist:



Flags (strings)










File.open path, File::RDONLY
File.open path, 'w'
File.open path, File::WRONLY|File::TRUNC|File::CREAT
File.open path, File::WRONLY|File::TRUNC|File::CREAT, '0666'
January 13, 2009
6 thanks

Universal partial

polymorphic_url is very useful if you want to create an universal partial that works for more than 1 type of object passed to it.

For example in you sidebar you might have a _sidebar.html.erb partial that’s supposed to display links to “Edit” and “Delete” actions. You can write it in such a way that it can be reused for different types of objects (in the example below we pass either a Post or a Note).


<%= render :partial => 'shared/sidebar', :locals => { :obj => Post.new -%>


<%= render :partial => 'shared/sidebar', :locals => { :obj => Note.new -%>


<%= link_to "Edit", polymorhpic_url(obj, :action => 'edit') -%>
<%= link_to "Delete", polymorphic_url(obj), :method => :delete -%>