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This commit vastly reduces the impact of auto explain logging when enabled, while keeping a negligible cost when disabled. The first implementation was based on the idea of subscribing to "sql.active_record" when needed, and unsubscribing once done. This is the idea behind AR::Relation#explain. Subscribe, collect, unsubscribe. But with the current implementation of notifications unsubscribing is costly, because it wipes an internal cache and that puts a penalty on the next event. So we are switching to an approach where a long-running subscriber is listening. Instead of collecting the queries with a closure in a dedicated subscriber, now we setup a thread local. If the feature is disabled by setting the threshold to nil, the subscriber will call a method that does nothing. That's totally cheap.
== Welcome to Rails
Rails is a web-application framework that includes everything needed to create
database-backed web applications according to the {Model-View-Controller (MVC)}[http://en.wikipedia.org/wiki/Model%E2%80%93view%E2%80%93controller] pattern.
Understanding the MVC pattern is key to understanding Rails. MVC divides your application
into three layers, each with a specific responsibility.
The View layer is composed of "templates" that are responsible for providing
appropriate representations of your application's resources. Templates
can come in a variety of formats, but most view templates are \HTML with embedded Ruby
code (.erb files).
The Model layer represents your domain model (such as Account, Product, Person, Post)
and encapsulates the business logic that is specific to your application. In Rails,
database-backed model classes are derived from ActiveRecord::Base. Active Record allows
you to present the data from database rows as objects and embellish these data objects
with business logic methods. Although most Rails models are backed by a database, models
can also be ordinary Ruby classes, or Ruby classes that implement a set of interfaces as
provided by the ActiveModel module. You can read more about Active Record in its
{README}[link:/rails/rails/blob/master/activerecord/README.rdoc].
The Controller layer is responsible for handling incoming HTTP requests and providing a
suitable response. Usually this means returning \HTML, but Rails controllers can also
generate XML, JSON, PDFs, mobile-specific views, and more. Controllers manipulate models
and render view templates in order to generate the appropriate HTTP response.
In Rails, the Controller and View layers are handled together by Action Pack.
These two layers are bundled in a single package due to their heavy interdependence.
This is unlike the relationship between Active Record and Action Pack which are
independent. Each of these packages can be used independently outside of Rails. You
can read more about Action Pack in its {README}[link:/rails/rails/blob/master/actionpack/README.rdoc].
== Getting Started
1. Install Rails at the command prompt if you haven't yet:
gem install rails
2. At the command prompt, create a new Rails application:
rails new myapp
where "myapp" is the application name.
3. Change directory to +myapp+ and start the web server:
cd myapp; rails server
Run with <tt>--help</tt> for options.
4. Go to http://localhost:3000 and you'll see:
"Welcome aboard: You're riding Ruby on Rails!"
5. Follow the guidelines to start developing your application. You may find the following resources handy:
* The README file created within your application.
* The {Getting Started with Rails}[http://guides.rubyonrails.org/getting_started.html].
* The {Ruby on Rails Tutorial}[http://railstutorial.org/book].
* The {Ruby on Rails Guides}[http://guides.rubyonrails.org].
* The {API Documentation}[http://api.rubyonrails.org].
== Contributing
We encourage you to contribute to Ruby on Rails! Please check out the {Contributing to Rails
guide}[http://edgeguides.rubyonrails.org/contributing_to_ruby_on_rails.html] for guidelines about how
to proceed. {Join us}[http://contributors.rubyonrails.org]!
== Travis Build Status {<img src="https://secure.travis-ci.org/rails/rails.png"/>}[http://travis-ci.org/rails/rails]
== License
Ruby on Rails is released under the MIT license.