Sunday, June 24, 2007

Code Digest #2

When you program for a living, you write lots of code. There is often some code that you are fond of. We started the Code Digest series to present such code written by the RHG developers. We encourage other teams and individual developers to share similar snippets in their blogs so we all can learn from each other and become better rails developers.

Mai Nguyen

Simple AJAX error messaging

When simple javascript validation is not enough and your product managers insist on ajax for error messaging, guess what. You have to implement ajax error messaging. One such case is 'username availability'. This simple example displays an error message on blur of a the username field. It doesn't hit the server unless the value of the field is well-formed (at least that will save you *some* network traffic ...)

Controller would look something like:
class FooController < ::ApplicationController

def is_username_taken

unless Person.find_by_username(params[:username])
render :nothing => true

message_html_id = params[:message_html_id]
render :update do |page|
page.replace_html message_html_id ,"Username is not available, please choose another."
# if you want error styling associated with the failed state
page << "document.getElementById('" + message_html_id + "').className = 'failed'"



View would look something like:
<dt><label>Username: </label></dt>
<dd id="username_input">
<%= text_field :person, :username, :class => "input", :type => 'text', :id => 'person_username_input' %>
<dd id="username_messaging"><%= @person.errors.on(:username)%></dd>

Your javascript would look something like:
var UserNameUnique = Class.create();
UserNameUnique.prototype = {
initialize: function( field_id, message_id) {
this.message_id = message_id;
this.field = document.getElementById( field_id );
if (typeof this.field == 'undefined') {return;}
// Observe blur on field
Event.observe(this.field, 'blur', this.checkName.bindAsEventListener(this));
checkName: function() {
if( typeof this.field != 'undefined' )
var name = this.field.value;
// don't hit server unless username is well-formed
var re = /^[A-Za-z])[a-zA-Z0-9]{2,25}$/;
if( name.match( re ))
new Ajax.Request('/foo/is_username_taken', {asynchronous:true, parameters:'username='+name+'&message_html_id='+this.message_id});

new UserNameUnique('person_username_input', 'username_messaging');

You could also use Rails helper observe_field instead of the writing your own javascript, but there is more flexibility in writing your own javascript (such as the need to check well-formed values before hitting the server).

Mark Brooks

Creating the options list for select tags can be annoying, especially if there is a requirement that the current "option" be displayed as the default option. With javascript, it isn't such a big deal, but one must take care of the degraded case as well.

In any event, the base object is an array of two-item hashes representing both the name of a video channel and the number of videos in that channel. By way of example:
@channelinfo = [
{"name"=>"Diabetes", "count"=>1},
{"name"=>"Pregnancy", "count"=>11}

The option values are the name fields of each hash. The key is, we want the current option value to be the first one in the option list, and the rest to be in alpha order by name.

So let's create an option builder from the bottom up.

First, we only need the channel names for the select tag. This gives us what we need:
options = @channelinfo.collect { |channel| channel['name'] }

That gives us a list of channel names. Using the list above, it would be ["Fitness", "Diabetes", "Pregnancy"].

However, we need to make sure that the current channel is first in the list. Let's say that current channel is Diabetes. Since we already have that value in @current_channel, we can exclude it from our options list:
options = @channelinfo.collect do |channel|
end.reject do |channelname|
channelname == @current_channel

Now the resulting list will look like ["Fitness", "Pregnancy"]. However, we still need the current channel to be at the front of the lift, so we add it back as the first element:
options = @channelinfo.collect do |channel|
end.reject do |channelname|
channelname == @current_channel

Now our options list looks like ["Diabetes", "Fitness", "Pregnancy"].

Two points. First, we want to make sure that, while the current channel is at the head of the list, the rest of the list items are in alpha order, so we add a sort directive in the appropriate place. Also, it is probably a good idea to exclude any nils that might pop up in the collection on the original data object, since it comes from a service and it is possible, however unlikely, that a hash might get spit out without a 'name' property. The more rigorous code looks like this now:
options = @channelinfo.collect do |channel|
end.compact.reject do | channelname |
channelname == @current_channel

Now we can generate our options list using:
options.collect do |channelname|
"<option>" + channelname + "</option>"

although if you want to, you can simply combine the whole thing as follows:
@channelinfo.collect do |channel|
end.compact.reject do |channelname|
channelname == @current_channel
end.sort.unshift(@current_channel).collect do |channelname|
"<option>" + channelname + "</option>"

to get the same result, and eliminate the unnecessary options binding.

Todd Fisher

Here is an extension trying to execute a block multiple times before giving up. Handy for network operations and such.

module Kernel
def could_fail(retries = 3, &block)
tries = 0
rescue Exception
tries += 1
if tries < retries

Call it as result = could_fail { some_operation_that_might_fail_first_time }

Todd Fisher

When you are writing a script that needs to auto install gems, you are likely to run into a problem that it stops because there are multiple platform versions available (jruby, win32, etc) and the gem command expects you to pick one that matches your platform. This patch forces to use a specific platform so no user interaction is needed. Original idea from Warren updated to support rubygems 0.9.4.

module GemTasks
def setup

return if $gems_initialized
$gems_initialized = true

# see =>
Gem::RemoteInstaller.class_eval do

alias_method :find_gem_to_install_without_ruby_only_platform, :find_gem_to_install

def find_gem_to_install( gem_name, version_requirement, caches = nil )
if caches # old version of rubygems used to pass a caches object
caches.each {|k,v| caches[k].each { |name,spect| caches[k].remove_spec(name) unless spec.platform == 'ruby' } }
find_gem_to_install_without_ruby_only_platform( gem_name, version_requirement, caches )
Gem::StreamUI.class_eval do

alias_method :choose_from_list_without_choosing_ruby_only, :choose_from_list
def choose_from_list( question, list )
result = nil
result_index = -1
list.each_with_index do |item,index|
if item.match(/\(ruby\)/)
result_index = index
result = item
return [result, result_index]


find_gem_to_install_without_ruby_only_platform( gem_name, version_requirement )




Val Aleksenko

Since class-level instance variable are not inherited by subclasses, you need to go some extra steps when writing a plugin using them. Depending on the amount of such variables, I have been either defining a method instead of class-level instance variables or forwarding them to subclasses.

Example #1. acts_as_readonlyable needs to provide a single class level instance variable. Defining a method instead.

def acts_as_readonlyable(*readonly_dbs)

def define_readonly_model_method(readonly_models)
(class << self; self; end).class_eval do
define_method(:readonly_model) { readonly_models[rand(readonly_models.size)] }

Example #2. acts_as_secure uses a bunch of variables. Forwarding them to subclasses.

def acts_as_secure(options = {})
@secure_except = unsecure_columns(options.delete(:except))
@secure_storage_type = options.delete(:storage_type) || :binary
@secure_class_crypto_provider = options.delete(:crypto_provider)
@secure_crypto_providers = {}

module ActsAsSecureClassMethods
def inherited(sub)
[:secure_except, :secure_storage_type, :secure_class_crypto_provider, :secure_crypto_providers].each do |p|
sub.instance_variable_set("@#{ p }", instance_variable_get("@#{ p }"))


Anonymous said...

Mai Nguyen,

You can clean up that RJS a little, ie:

render :update do |page|
page[message_html_id].replace_html "Username is not..."
page[message_html_id].class_name = 'failed'

Unknown said...

How do you prevent a DNS attack when checking that a username is available? In addition, a hacker could use that to determine all of the usernames that are currently registered on the site. There needs to be some sort of throttling.