How to Install ‘therubyracer’ or ‘libv8’ gem(s) on OSX

Recently, I need to move some Rails projects I was working on to new computer and this needs me to install all the dependencies for these projects.  While using bundler to install the gems; I encountered the following error:

extconf failed, exit code 1
Gem files will remain installed in /Users/ash/.rvm/gems/ruby-2.2.1/gems/libv8-3.16.14.3 for inspection.
Results logged to /Users/ash/.rvm/gems/ruby-2.2.1/extensions/x86_64-darwin-14/2.2.0-static/libv8-3.16.14.3/gem_make.out

An error occurred while installing libv8 (3.16.14.3), and Bundler cannot continue.
Make sure that `gem install libv8 -v '3.16.14.3'` succeeds before bundling.

Fortunately, with homebrew fixing this (on OSX 10.11, El Capitan at least) worked perfectly. Simply execute these commands:

brew install v8
gem install therubyracer
gem install libv8 -v '3.16.14.3' -- --with-system-v8

Estabilishing ActiveRecord Database Connections in Ruby (But Without Rails)

Anyone who has even the smallest amount of experience developing with Ruby on Rails knows that Rails has some pretty sweet configuration conventions which make switching between environments very easy. Switching from development to production to testing is as easy as changing the RAILS_ENV variable. No doubt Rails does some dark magic behind the scenes to trivialise this. But what if you’re writing an app in Ruby, without the Rails to guide you?

ActiveRecord 101

Establishing a database connection in ActiveRecord without Rails is pretty basic:

require 'active_record'

ActiveRecord::Base.establish_connection({
  adapter:  'sqlite3',
  database: 'db/test.sqlite3'
})

It establishes a connection to the specified database with the specified connection  configuration. If you were using MySQL or PostgreSQL you would provide the relevant configuration such as username, password and host. With the connection established, we can now start consuming the connection however we like. For example, we may want to use funky Rails’ style models (so we can easily create and populate tables and reference them):

ActiveRecord::Schema.define do
  create_table :things do |t|
    t.integer :id, :null => false
    t.string  :name
  end
end

class Thing < ActiveRecord::Base
end

Thing.create({
  id:   0,
  name: 'Broomstick'
})

Care about the Environment

The other magic in Rails is the environment selection. The main benefit with this is that we can have isolated connection configurations and have our Ruby app automatically select the correct settings for us (using our environment variable ENV).

require 'active_record'

conf = case ENV['DB']
when 'conf1'
{
  adapter: 'sqlite3',
  database: 'db/mydb1.sqlite3'
}
when 'conf2'
{
  adapter: 'sqlite3',
  database: 'db/mydb2.sqlite3'
}
else
  raise 'export DB=conf[n]'
end

ActiveRecord::Base.establish_connection conf

Mind you, before this will work you first need to set the environment variable:

export DB=conf1 # or conf2

Now, depending on ENV[‘DB’], the code will open a connection to the corresponding database.

Best Practises

It’s never a good idea to store sensitive information (like database connection information and usernames and password of any kind) in your code base. Rails uses an external database configuration file called database.yml to solve this problem, so lets implement one ourselves.

database.yml:

conf1:
  adapter: sqlite3
  database: db/mydb1.sqlite3

conf2:
  adapter: sqlite3
  database: db/mydb2.sqlite3

Loading this YAML file is very simple:

require 'active_record'
conf = YAML.load_file('database.yml')
ActiveRecord::Base.establish_connection conf[ENV['DB']]

Now we can set our environment variable same as before, but our code will look in a YAML file for the proper database configuration automatically, without having to store database connection information directly inside our code.

Association Cardinality in Rails

From time to time I’ve noticed people who struggle with cardinality and associations in Ruby on Rails. So, I thought I would attempt to create a cheat sheet here to help developers understand relationship cardinality and how it maps to associations.

ActiveRecord can be used to describe relations with one-to-one, one-to-many and many-to-many cardinality; where each model defines its relation to another. Let’s cover each of the three types of associations.

One-to-one

Use `has_one` in the base and `belongs_to` in the association:

class Family < ActiveRecord::Base
  has_one :home
end
class Home < ActiveRecord::Base
  belongs_to :family
end

A common question about a one-to-one association is ‘how to know which direction the has_one and belongs_to go?’  The correct way to know, is that whichever model has the foreign key, gets the `belongs_to`.  In this case, Home has the foreign key `family_id`.

One-to-one relationships are a bit odd, and as a general rule, if you find yourself using a lot of them, there is probably a better solution.

One-to-many

Use `has_many` in the base and `belongs_to` in the association:

class Family < ActiveRecord::Base
  has_many :parents
end
class Parent < ActiveRecord::Base
  belongs_to :family
end

This will be your most common relationship. As with one-to-one’s, the table with the foreign key gets the `belongs_to` (although this is a lot more obvious with a one-to-many). In this case the foreign key is `family_id`.

Many-to-many

These can be a lot more complicated and there is actually a couple of different ways to do it.

The first way involves a specific joining model. This results in 2 stages of has_many associations. It is referred to as `has_many :through` and is primarily used if you need to fully control the joining model/table:

class Family < ActiveRecord::Base
  belongs_to :parent
  belongs_to :kid
end
class Parent < ActiveRecord::Base
  has_many :kids, through: :families
end
class Kid < ActiveRecord::Base
  has_many :parents, through: :families
end

The second (and my preferred way) is to use the `has_and_belongs_to_many` method:

class Parent < ActiveRecord::Base
  has_and_belongs_to_many :kids 
end
class Kid < ActiveRecord::Base
  has_and_belongs_to_many :parents
end

The main difference (or disadvantage) with the `has_and_belongs_to_many`method is that the intermediary joining table and foreign keys need to be exactly named to match what Rails expects. Which many-to-many method you use ultimately depends on whether you need to work with the relationship model as its own entity directly.

MySQL utf8mb4 Encoding Breaks ActiveRecord’s Schema Setup

I recently wrote about the virtues of true UTF8 (utf8mb4) character sets in MySQL and how to change your database to use it. Today we will discuss a possible problem you may encounter when you do when programming on Ruby on Rails. The error looks something like this:

$ rake db:setup

Mysql::Error: Specified key was too long; max key length is 767 bytes:
CREATE UNIQUE INDEX unique_schema_migrations ON schema_migrations (version)

The problem exists because the utf8mb4 character set uses the full 4 bytes per character rather than the 1-3 of UTF8 (the character set most people mistakenly use thinking they’ll have full Unicode compliance). Because of this extra size, the schema_migration may no longer fit.

This small patch will set default mysql string column length to 191 instead of 255 which is the new index limit on utf8mb4 (aka real utf8).

# config/initializers/mysqlpls.rb
require 'active_record/connection_adapters/abstract_mysql_adapter'

module ActiveRecord
  module ConnectionAdapters
    class AbstractMysqlAdapter
      NATIVE_DATABASE_TYPES[:string] = { :name => "varchar", :limit => 191 }
    end
  end
end

 

‘belongs_to’ and ‘has_one’ Differentiated

One of the more common confusions with ActiveRecord associations is the difference between the `has_one` and `belongs_to` relationships.

However, they are not interchangeable, and there are some serious differences. A `belongs_to` can only go in the class that holds the foreign key whereas `has_one` means that there is a foreign key in another table that references this class. So `has_one` can only go in a class that is referenced by a column in another table.

So this is wrong:

class Transaction < ActiveRecord::Base
  # The transactions table has a order_id  
  has_one :order                
end

class Order < ActiveRecord::Base
  # The orders table has a transaction_id
  has_one :transaction          
end

So is this:

class Transaction < ActiveRecord::Base
  # The transactions table has a order_id
  belongs_to :order             
end

class Order < ActiveRecord::Base
  # The orders table has a transaction_id
  belongs_to :transaction     
end

For a two-way association, you need one of each, and they have to go in the right class. Even for a one-way association, it matters which one you use, and which direction you use it:

class Transaction < ActiveRecord::Base
  # This table has no foreign keys
  has_one :order             
end

class Order < ActiveRecord::Base
  # The orders table has a transaction_id
  belongs_to :transaction     
end

Adding Wiki Style Functionality to Your Rails Site Using 'acts_as_versioned'

If you need to add basic wiki style functionality to your Ruby on Rails models, there is a really easy way to get similar model versioning without having to resort to cutting the code yourself.

The acts_as_versioned ‘plugin’ has been available for quite some time, but its been made far better by it now becoming a gem instead of an old-school plugin. The authors have gone to considerable effort to make it as painless as possible to use.

This post, is designed to give you a brief over-view into how to get up and running with with models which ‘acts_as_versioned’.  Because its the current version (at time of posting) and because its awesome, this walk-though assumes that you are using Rails 3, not 2.  The instructions for Rails 2 sites are similar, but you’ll need to tweak this for it to work.

First, you need to grab the gem:

sudo gem install acts_as_versioned

Next, add the dependency to the ‘Gemfile’, it doesn’t matter too much where it goes, I stuck it somewhere in the middle:

gem 'acts_as_versioned', '0.6.0'

Next, just under the ‘ActiveRecord::Base’ line in the model’s class file, instruct the class that its to act as a versioned model.

class Article < ActiveRecord::Base
  acts_as_versioned
end

Then, in the migration file you need to execute the model's method to create the version table.  This is key because the acts_as_versioned gem actually creates an additional database table to house all the previous versions of a given record.  Obviously, you need to delete the table is the schema is taken down.  My migration now looks like:

class CreateArticles < ActiveRecord::Migration
  def self.up
    create_table :article do |t|
      t.string :title
      t.string :body
      t.integer :user_id

      t.timestamps
    end
    Article.create_versioned_table
  end

  def self.down
    drop_table :articles
    Article.drop_versioned_table
  end
end

The key method is the

Article.create_versioned_table

which creates the version table of the model. Now, get rake to create the database:

 rake db:migrate

Thats it!  Its done.  Using acts_as_versioned is simple. I'll provide some examples, where '@article' represents an instance of a model setup 'acts_as_versioned'. To find the current version of an article you can use the version property:

@article.version 

But just performing a normal ActiveRecord lookup returns the most current version anyway, so to revert to a previous version use the revert_to method on an article instance:

@article.revert_to(version_number)

You can save (just like you've done a hundred times before) a previous version as the current on by using the save method. The save on a reverted articles will just create a new version.

To get the number of versions:

@article.versions.size

Since '@article.versions' returns an array of versions, you can do neat things like this:

History

<% for version in @article.versions.reverse %> Version <%= version.version %> <%= link_to '(revert to this version)', :action => 'revert_to_version', :version => version.id, :id => @article %>
<% end %>

Obviously for this to work, you'd need to create a 'revert_to_version' action in the appropriate controller, but you get the idea.

acts_as_versioned is an amazing piece of work, and aside from the wiki-like functionality it gives you for very little effort, I can imagine scenarios such as audit and trace logs and "undo" features which could really benefit from this little gem.

Using Rails’ Flash Messages with AJAX Requests

Have you ever wondered how to get access to the Ruby on Rails‘ flash message when performing a AJAX or restful web request?  You might hit yourself on the head when you discover how easy it is.  Simply append the flash message to the Response headers.  You could even wrap this in a helper, and using an after_filter to automatically add the header for you on every AJAX response.

class ApplicationController < ActionController::Base
after_filter :flash_headers

def flash_headers
  # This will discontinue execution if Rails detects that the request is not
  # from an AJAX request, i.e. the header wont be added for normal requests
  return unless request.xhr?

  # Add the appropriate flash messages to the header, add or remove as
  # needed, but I think you'll get the point
  response.headers['x-flash'] = flash[:error]  unless flash[:error].blank?
  response.headers['x-flash'] = flash[:notice]  unless flash[:notice].blank?
  response.headers['x-flash'] = flash[:warning]  unless flash[:warning].blank?

  # Stops the flash appearing when you next refresh the page
  flash.discard
end

And then you just read the header with whatever you happen to be reading it with. For completeness sake here is an example of how to read the header in JavaScript using Prototype:

 new Ajax.Request('/your/url', {
  onSuccess: function(response) {
    var flash = response.getHeader('x-flash);
    if (flash) alert(flash);
 }
});

How To Install Phusion Passenger on a CentOS/cPanel Box

The Usual Disclaimer
Please beware that even if these instructions work, that they may break your setup.  While these worked fine for me, you should take extreme care applying them to your situation.  Follow these instructions at your own risk. They work fine for me on a CentOS 4.6 box with cPanel 11.23.4-R26138 and WHM 11.23.2.

First off, Phusion Passenger only works with Apache 2.x so if your server doesn’t have that thats the next thing you’ll have to do.  T o upgrade Apache, go to your main control panel at https://server.ip.address:2087/ and click the “Apache Update” link on the left.  If given the option, I highly recommend that you choose Apache 2.2 instead of 2.0.

Next, download Passenger. Assuming you already have Ruby and RubyGems installed on your server, simply run (as root) gem install passenger. This will pull the passenger library and code down to the server and place it with your other RubyGems.  It’s important to know that this does not install Passenger into Apache, so obviously its non-operational.  If you don’t have Ruby and RubyGems, you’re way too far down the line with reading this article and need to get up to speed with actually getting those on to your box.  Google can help with that!

At this point it’s probably useful to have the Passenger User Guide up on screen, just for reference.

Third, you need to compile and install the Passenger module within Apache. This sounds worse than it is, but before you do it, it is important to set a couple of environment variables to make it work properly.  In your SSH console type thrse two commands into it:

export APXS2=/usr/local/apache/bin/apxs
export APR_CONFIG=/usr/local/apache/bin/apr-1-config

Next, run:

passenger-install-apache2-module

It might take a few minutes, but if everything goes well you’ll eventually end up with Passenger telling you to add a few lines to your Apache configuration file (the code you get may differ slightly):

LoadModule passenger_module /usr/local/lib/ruby/gems/1.8/gems/passenger-2.0.2/ext/apache2/mod_passenger.so
PassengerRoot /usr/local/lib/ruby/gems/1.8/gems/passenger-2.0.2
PassengerRuby /usr/local/bin/ruby

However, despite what it tells you, do not put it in the httpd.conf file – cPanel can rewrite that file and your changes can be lost breaking your passenger configuration and virtual site.  Instead, add it to /usr/local/apache/conf/includes/pre_virtualhost_global.conf – this file might not exist until you make it, but that’s okay.

Lastly restart Apache and if you didnt get any critial errors, you should be in business.  I have had issue on other servers where the Rails app failed to load because the RubyGems version was too old, so keep your eye on the error_log file if Apache starts but it still doesn’t work.  Also refer to the Passenger User Guide for further configuration and usage information.

Solve uninitialized constant ApplicationController in Rails 2.3

So I am casually writing my Ruby on Rails application, like I have a million times before on the newest version of Rails (which at time of writing, was version 2.3).  So I open up an blank project with some code I use in all my Rails apps, and copy over the relevant files, and replace the application_crontroller.rb with application.rb.

But as it turns out, there were some major changes with 2.3, not the least being that the application controller is no longer called application.rb Now it’s referred to as application_controller.rb.

So it’s easy to fix, just rename the file or, it you’re into scripty goodness, run:

rake rails:update

Be sure to update your server stack before deploying. Obviously if you rename the file to get it working on Rails 2.3 and upload to a Rails 2.2 server you’ll be in trouble. Don’t forget that Rails 2.3 also requires a new version of Phusion Passenger!