When you’re developing an application in Rails (or Ruby), you spend lots of time in the IRB, the Interactive Ruby Shell. Usually just to test some Ruby code, start an application console or debug something going on inside the project. Yesterday, looking at a coworker screen, I saw he had his console with lots of color hints, and I thought it was pretty nice. I asked him about that and he told me he was using a special gem for that.
The gem is called wirble. It has some nice defaults and allows you to configure the colors as you wish. To use it in your consoles, just add this lines to your ~/.irbrc file:
begin
require 'rubygems'
require 'wirble'
Wirble.init
Wirble.colorize
rescue LoadError => err
warn "Couldn't load Wirble: #{err}"
end
Of course, you’ll need to install the gem. Just issue a gem install wirble and you’ll be good to go!
If you ever try to run a GUI Java application when using xmonad as the Window Manager, you’ll probably end up with a nice flat grey window where your buttons, toolbars and other desktop GUI goodies should be. I ran into that problem the other day when trying to evaluate the RubyMine Ruby on Rails IDE from which I heard such good things. After a rather painful installation of the official Java 6 JDK from Oracle in Ubuntu Lucid Lynx (which I’ll write about in some other time), I managed to start up RubyMine just to find out I was seeing absolutely nothing on the screen.
I Googled a bit and essentially I found out that the problem was with the Java GUI Toolkit rather than RubyMine, because the framework relies on a list of window managers to handle some things, and xmonad is not in that list.
So since I was not using OpenJDK, I opted for the setWMName option and added this final line into my xmonad.hs file:
xmonad $ defaultConfig
{ terminal = myTerminal
, workspaces = myWorkspaces
, keys = keys'
, modMask = modMask'
, layoutHook = layoutHook'
, manageHook = manageHook'
, logHook = myLogHook dzenLeftBar >> fadeInactiveLogHook 0xdddddddd
, normalBorderColor = colorNormalBorder
, focusedBorderColor = colorFocusedBorder
, borderWidth = 2
, startupHook = setWMName "LG3D"
}
Setting the window manager name to LG3D will trick the Java GUI Toolkit and the application will be painted correctly.
Enjoy your Java applications with the power of xmonad!
Sometimes you need to use multiple databases in your Rails projects. Usually when some data must be shared between different applications. When this happens you usually have some models in a shared database, and some other models in the specific application database. This can be easily done using the establish_connection method in the shared models to tell them they have to connect to a different database.
However, when you need some interaction between those shared models and the models of your specific application, like a has_many, :through association, some problems arise. The typical Many To Many association uses an intermediate database table that links the relation between two models, and allows you to add some extra information on that relation. When navigating through the association, Rails tries to make an SQL query that joins the model with this intermediate table. For example, imagine you have a Team model, which has many Players, but a player can also be on more than one team. We use an intermediate model TeamPlayers (and we can also use it to save the role of that player into that team, for example). You would have those three tables:
- teams
- players
- teams_players
When asking for the players of a given Team, Rails would do something similar to this:
SELECT "players".* FROM "players" INNER JOIN teams_players" ON "players".id = "teams_players".player_id WHERE "players".team_id = 1
Where 1 is the id of the team you asked for. This [obviously] works perfectly fine when everything is in the same database, and it’s as efficient as the SQL database manager you’re using. What happens, however, when we have the Player model in another database? It will miserably fail because Rails will try to join with a table that doesn’t exist.
Unfortunately, there’s no efficient way to solve this problem, that is, using SQL, as you can’t work with tables from different databases. However, there’s a rather elegant solution that Brian Doll cared to implement as a gem a while ago. As indicated in the GitHub readme, you just have to use a has_many_elsewhere relation instead of the usual one, and make sure that the model referenced has the connection established to the shared database. And that’s all.
The magic donde behind the scenes is pretty simple: this gem just replicates the same methods that the ActiveRecord::Base class does in the has_many method call, changing the failing unique SQL calls to double SQL calls, one for each database, fetching the intermediate models first, and then fetching the remote models using those ids.
This method is not perfect, as probably not all the goodness of the original association can be done with it, but for simple scenarios is more than enough.
One thing I find fascinating about Ruby is the fact that most common tasks are already programmed for you in its library. The Enumerable module is a clear example of that, providing you with lots of functionality to manipulate collections of objects.
One of those useful methods I discovered the other day was each_slice. This method allows you to iterate over the collection, just as each does, but lets you do it changing how many elements of the collection you get on each iteration. This is the example you can get from the documentation page:
(1..10).each_slice(3) {|a| p a}
# outputs below
[1, 2, 3]
[4, 5, 6]
[7, 8, 9]
[10]
You can see that from the original array from 1 to 10, on every iteration Ruby prints the numbers in groups of three, and the last one alone since the collection is not a multiple of 3. Now think about having to do this manually: it’s not that hard, but its error prone and you have to do all that typical arithmetic logic that should be easy but never is. How handy that Ruby has already done that job for you.
This method is also pretty useful when working in Ruby on Rails. One simple example is when you have to manually implement some kind of pagination, or show a list of elements in columns or rows of fixed size: you have to simply iterate with each_slice and put the page/row/column logic on the block, and voilà.
I strongly suggest you take a look at the Enumerable module reference to take a look at all the other flavours of each methods it has and I’m sure you’ll find all of them pretty useful in lots of situations!
If you ever wondered how to easily retrieve a random record in an ActiveRecord model, here’s en easy way to do that: use the sample method.
sample is a class method from the Array class that retrieves one or more random items from an array instance. It conveniently returns nil or an array lesser than the items requested if the original array has not enough items.
Since all ActiveRecord finds return an array, you can easily add the sample call at the end of a find call to get the random items you need from complex queries.
For example, imagine you have a Book model which has a published_at datetime attribute and you want to show, in your library home page, a random selection of 5 books that have been published. You can easily get those using this snippet:
def get_random_books_for_home_page
find(:all).sample(5)
end
If you’ve ever used the serialize method in an ActiveRecord model, you may have faced the problem of writing a fixture for that particular serialized field. The serialize method is quite handy if you need to store a complex object (like a Hash or an Array) in a database field, without having to create additional models and relationships between them. A good example could be store the preferences of a user in a hash:
class User < ActiveRecord::Base
serialize :preferences, Hash
end
This way you can set and get the preferences attribute of the user without having to worry much about it:
User.first.preferences # => {:show_email => true, :allow_pm => :false, ...}
user = User.first
user.preferences = {:show_email => true, :allow_pm => :false}
user.save
In order to do that, at the database table for the User class, among the other fields you’ll need to add a text column for the preferences. After that, you can easily work with your model without too much of a hassle.
If you care to look at the rows of the model’s table, you’ll see that the serialized attribute is stored in YAML format:
sqlite> select preferences from users where id = 1;
---
:show_email: false
:allow_pm: false
Now, that if you need to add a fixture for your model that needs the preferences attribute to be set to a particular value? If you have to manually parse the value you want to put in there to be tested, it’ll be a pain in the ass. Even if you have a script that converts certain values to YAML so you can copy and paste the script output to the fixture, it’s not very comfortable. Hopefully, you have to remember that you can use some ERB magic in your fixtures 
So, here’s the deal if you need to add something to the serializable attribute:
one:
preferences: <%= {:show_email => true, :allow_pm => :false}.to_yaml.inspect %>
The to_yaml method converts the Hash into a YAML representation, and with inspect we convert that to a string. Using this technique, we can add whatever we want to a serialized field in our fixtures.
Today I was working on a web site that needs to retrieve some RSS feed over the internet. Since the web page has no server (HTML + javascript only) I couldn’t access the feed from the server side. Also, because of the Cross Domain limitation of Ajax requests, I couldn’t access the RSS in the client either. I searched Google for an API and found the Google Feed API, which does exactly what I want. However, because (I think) Google caches the RSS feed you request, there was a significant delay (about half an hour) between the update of the RSS contents and the RSS provided by Google (the feed was updated in a per minute basis, as it was a CoverItLive event). Seeing I couldn’t access really recent posts from the feed using Google, I decided to implement my own RSS API using JSONP in a ruby on rails environment, since having an external server act as a proxy was allowed for the overall solution.
The tools I needed I got from those two websites: http://rubyrss.com/ for a RSS parser, and http://blogs.sitepoint.com/2006/10/05/json-p-output-with-rails/ on how to build a simple JSONP response on the server side.
Basically you have to start creating a new controller that will handle the JSONP requests. In my case I just added a ‘Feed’ controller:
$ script/generate controller Feed
Then you edit the app/controllers/feed_controller.rb file and start coding. We will assume that the request will come in this form: http://server/feed/get?callback=some_callback&url=the_url_of_the_feed. Having this information, the controller code is pretty straightforward.
class FeedController < ApplicationController
require 'rss/1.0'
require 'rss/2.0'
def get
begin
url_contents = Net::HTTP.get(URI.parse(params[:url]))
rss = RSS::Parser.parse(url_contents, false)
json = { "error" => false, "feed" => rss }.to_json
rescue
json = { "error" => true }.to_json
end
respond_to do |format|
format.js { render_json json }
end
end
def render_json(json)
callback = params[:callback]
response = begin
if callback
"#{callback}(#{json});"
else
json
end
end
render({:content_type => :js, :text => response})
end
endThe first two lines are the requirements for the RSS module, which will allow us to parse a RSS feed. After that, we start with the get request. In there, we use the Net::HTTP.get() method, which will retrieve a URL content using a GET request and return its contents. To do so, we need to pass it an uri parameter, which we can get from the entire URL using the method URI.parse(). After this call, we have the XML of the RSS feed in url_contents. What we have to do now is build an RSS object with this XML. We’ll do that by calling RSS::Parser.parse(). If you wish to do some modifications to the RSS contents, now is your change. In this simple example we’ll simply bulk it all to the response.
To build the response, we need a JSON object. If everything went as expected, we can create a JSON object by simply creating a ruby associative array and calling the to_json method on it:
json = { "error" => false, "feed" => rss }.to_json
If, on the contrary, we got an error (bad URL, bad RSS, whatever), we simply return the same JSON object with the error property set to true (that’s done in the rescue clause).
After we have this JSON object built, we simply have to output the results. To do so, we use the help of a method called render_json which we have added to the controller code. In this method we simply output the JSON if we provide no callback (this means no JSONP), or either a padded JSON (hence the name JSONP) with the callback name followed by the JSON data. In either case we render the results as a js type.
For more detailed information on how JSONP works, check http://en.wikipedia.org/wiki/JSON#JSONP, but what you basically need to know is that when you do a JSONP request what you’re really doing is retrieve a chunk of javascript code that will be run on your client, so be aware of the security issues you can have here.
One of the interesting new things on the .NET platform is the recent addition of Python and Ruby to the CLR. Both versions for .NET are called IronPython and IronRuby respectively, and they provide some new and good things to the platform.
Python and Ruby lovers will see now that they can use all the library and features of the .NET platform programming in their favorite scripting language. Since both of them are object oriented, you can now write fully fledged apps using either of them.
However, there’s another interesting application for IronPython and IronRuby: adding scripting support for your existing .NET applications. This can be a very useful and powerful way to extend your applications and give the user freedom to program their own mini programs, scripts or whatever in your applications. It could be good for defining rules, assigning and calculating values, etc.
I’ll provide a simple class you can use to add scripting to your application. I’ll use IronPython in this example.
First of all, you have to download IronPython and install it, and add the references to the assemblies on your project references.
The usual way to proceed in those cases is to provide the user of some local variables you give them access to, execute the script, and then recover the values of those or new variables. To do so, You can use a class similar to this one:
using System;
using System.Collections.Generic;
using System.Text;
using IronPython.Hosting;
using Microsoft.Scripting.Hosting;
using Microsoft.Scripting;
namespace Scripting
{
internal class PythonEngine
{
ScriptEngine m_engine;
ExceptionOperations m_exceptionOperations;
SortedDictionary<string, object> m_inputVariables;
string m_script;
internal PythonEngine()
{
m_engine = Python.CreateEngine();
m_exceptionOperations = m_engine.GetService<ExceptionOperations>();
}
internal SortedDictionary<string, object> ScriptVariables
{
set { m_inputVariables = value; }
}
internal string Script
{
set { m_script = value; }
}
internal ExceptionOperations ExceptionOperations
{
get { return m_exceptionOperations; }
}
internal SortedDictionary<string, object> Execute()
{
//Create structures
SourceCodeKind sc = SourceCodeKind.Statements;
ScriptSource source = m_engine.CreateScriptSourceFromString(m_script, sc);
ScriptScope scope = m_engine.CreateScope();
//Fill input variables
foreach (KeyValuePair<string, object> variable in m_inputVariables)
{
scope.SetVariable(variable.Key, variable.Value);
}
SortedDictionary<string, object> outputVariables = new SortedDictionary<string, object>();
//Execute the script
try
{
source.Execute(scope);
//Recover variables
foreach (string variable in scope.GetVariableNames())
{
outputVariables.Add(variable, scope.GetVariable(variable));
}
}
catch (Exception e)
{
string error = m_exceptionOperations.FormatException(e);
//Do something with the pretty printed error
throw;
}
return outputVariables;
}
}
}
Usage of this class is pretty simple. You have to provide the object the script you want to execute and the input variables the script will have available as local variables. Once this is done, you have to call the Execute method, and this method will either return the output variables of the execution of the resulting script, or throw an exception.
Well, this is my first post after holidays and it won’t be very long.
Imagine you are developing a rails application. Usually you have:
- a terminal with the server to see what petitions are received.
- a terminal with a tail of development.log to see what happens with the database.
- a terminal with a tail of test.log if you are testing something.
This are a lot of windows… And the other day one friends was very happy and after asking for a while I discovered that the reason was the simple line showed above… With only one Ctrl+C you can kill all this processes
script/server & tail -f log/development.log & tail -f log/test.log & tail -f ; jobs -p | awk '{print "kill -2 " $0}' | sh
Maybe one of the best ways to learn a new programming language is playing with it. Nowadays if you don’t code in Ruby you aren’t cool. I have to recognize that programming in Ruby is funnier than in other language and for the moment I don’t have anything bad to say about it.
Well, I want to introduce Ruby Quiz. It is a collection of minigames prepared to learn Ruby (or to improve your skills). Every week they publish one game, and if you are brave you can send your solution to them and it will be public. I think it’s one of the best ways to learn because you can compare solutions and find were you are weak in Ruby and redo your solution doing it smarter.
