This guide is designed for beginners who want to get started with a Rails application from scratch. It does not assume that you have any prior experience with Rails. However, to get the most out of it, you need to have some prerequisites installed:

It is highly recommended that you familiarize yourself with Ruby before diving into Rails. You will find it much easier to follow what's going on with a Rails application if you understand basic Ruby syntax. Rails isn't going to magically revolutionize the way you write web applications if you have no experience with the language it uses. There are some good free resources on the net for learning Ruby, including:

Rails is a web development framework written in the Ruby language. It is designed to make programming web applications easier by making several assumptions about what every developer needs to get started. It allows you to write less code while accomplishing more than many other languages and frameworks. Longtime Rails developers also report that it makes web application development more fun.

Rails is opinionated software. That is, it assumes that there is a best way to do things, and it's designed to encourage that best way - and in some cases discourage alternatives. If you learn "The Rails Way" you'll probably discover a tremendous increase in productivity. If you persist in bringing old habits from other languages to your Rails development, and trying to use patterns you learned elsewhere, you may have a less happy experience.

The Rails philosophy includes several guiding principles:

Rails is organized around the Model, View, Controller architecture, usually just called MVC. MVC benefits include:

A model represents the information (data) of the application and the rules to manipulate that data. In the case of Rails, models are primarily used for managing the rules of interaction with a corresponding database table. In most cases, one table in your database will correspond to one model in your application. The bulk of your application's business logic will be concentrated in the models.

Views represent the user interface of your application. In Rails, views are often HTML files with embedded Ruby code that performs tasks related solely to the presentation of the data. Views handle the job of providing data to the web browser or other tool that is used to make requests from your application.

Controllers provide the "glue" between models and views. In Rails, controllers are responsible for processing the incoming requests from the web browser, interrogating the models for data, and passing that data on to the views for presentation.

Rails provides a full stack of components for creating web applications, including:

Action Controller is the component that manages the controllers in a Rails application. The Action Controller framework processes incoming requests to a Rails application, extracts parameters, and dispatches them to the intended action. Services provided by Action Controller include session management, template rendering, and redirect management.

Action View manages the views of your Rails application. It can create both HTML and XML output by default. Action View manages rendering templates, including nested and partial templates, and includes built-in AJAX support.

Active Record is the base for the models in a Rails application. It provides database independence, basic CRUD functionality, advanced finding capabilities, and the ability to relate models to one another, among other services.

Action Mailer is a framework for building e-mail services. You can use Action Mailer to send emails based on flexible templates, or to receive and process incoming email.

Active Resource provides a framework for managing the connection between business objects an RESTful web services. It implements a way to map web-based resources to local objects with CRUD semantics.

Railties is the core Rails code that builds new Rails applications and glues the various frameworks together in any Rails application.

Active Support is an extensive collection of utility classes and standard Ruby library extensions that are used in the Rails, both by the core code and by your applications.

The foundation of the RESTful architecture is generally considered to be Roy Fielding's doctoral thesis, Architectural Styles and the Design of Network-based Software Architectures. Fortunately, you need not read this entire document to understand how REST works in Rails. REST, an acronym for Representational State Transfer, boils down to two main principles for our purposes:

For example, to a Rails application a request such as this:

DELETE /photos/17

would be understood to refer to a photo resource with the ID of 17, and to indicate a desired action - deleting that resource. REST is a natural style for the architecture of web applications, and Rails makes it even more natural by using conventions to shield you from some of the RESTful complexities.

If you’d like more details on REST as an architectural style, these resources are more approachable than Fielding’s thesis:

If you follow this guide, you'll create a Rails project called blog, a (very) simple weblog. Before you can start building the application, you need to make sure that you have Rails itself installed.

In most cases, the easiest way to install Rails is to take advantage of RubyGems:

Open a terminal, navigate to a folder where you have rights to create files, and type:

This will create a Rails application that uses a SQLite database for data storage. If you prefer to use MySQL, run this command instead:

And if you're using PostgreSQL for data storage, run this command:

In any case, Rails will create a folder in your working directory called blog. Open up that folder and explore its contents. Most of the work in this tutorial will happen in the app/ folder, but here's a basic rundown on the function of each folder that Rails creates in a new application by default:

Just about every Rails application will interact with a database. The database to use is specified in a configuration file, config/database.yml. If you open this file in a new Rails application, you'll see a default database configuration using SQLite. The file contains sections for three different environments in which Rails can run by default:

Rails comes with built-in support for SQLite, which is a lightweight serverless database application. While a busy production environment may overload SQLite, it works well for development and testing. Rails defaults to using a SQLite database when creating a new project, but you can always change it later.

Here's the section of the default configuration file with connection information for the development environment:

If you don't have any database set up, SQLite is the easiest to get installed. If you're on OS X 10.5 or greater on a Mac, you already have it. Otherwise, you can install it using RubyGems:

If you're not running OS X 10.5 or greater, you'll need to install the SQLite gem. Similar to installing Rails you just need to run:

If you choose to use MySQL, your config/database.yml will look a little different. Here's the development section:

If your development computer's MySQL installation includes a root user with an empty password, this configuration should work for you. Otherwise, change the username and password in the development section as appropriate.

If you choose to use PostgreSQL, your config/database.yml will be customized to use PostgreSQL databases:

Change the username and password in the development section as appropriate.

One of the traditional places to start with a new language is by getting some text up on screen quickly. To do that in Rails, you need to create at minimum a controller and a view. Fortunately, you can do that in a single command. Enter this command in your terminal:

Rails will create several files for you, including app/views/home/index.html.erb. This is the template that will be used to display the results of the index action (method) in the home controller. Open this file in your text editor and edit it to contain a single line of code:

You actually have a functional Rails application already - after running only two commands! To see it, you need to start a web server on your development machine. You can do this by running another command:

This will fire up the lightweight Webrick web server by default. To see your application in action, open a browser window and navigate to http://localhost:3000. You should see Rails' default information page:

The "Welcome Aboard" page is the smoke test for a new Rails application: it makes sure that you have your software configured correctly enough to serve a page. To view the page you just created, navigate to http://localhost:3000/home/index.

You'd probably like to replace the "Welcome Aboard" page with your own application's home page. The first step to doing this is to delete the default page from your application:

Now, you have to tell Rails where your actual home page is located. Open the file config/routes.rb in your editor. This is your application's, routing file, which holds entries in a special DSL (domain-specific language) that tells Rails how to connect incoming requests to controllers and actions. At the bottom of the file you'll see the default routes:

The default routes handle simple requests such as /home/index: Rails translates that into a call to the index action in the home controller. As another example, /posts/edit/1 would run the edit action in the posts controller with an id of 1.

To hook up your home page, you need to add another line to the routing file, above the default routes:

This line illustrates one tiny bit of the "convention over configuration" approach: if you don't specify an action, Rails assumes the index action.

Now if you navigate to http://localhost:3000 in your browser, you'll see the home/index view.

Rails scaffolding is a quick way to generate some of the major pieces of an application. If you want to create the models, views, and controllers for a new resource in a single operation, scaffolding is the tool for the job.

In the case of the blog application, you can start by generating a scaffolded Post resource: this will represent a single blog posting. To do this, enter this command in your terminal:

The scaffold generator will build 13 files in your application, along with some folders, and edit one more. Here's a quick overview of what it creates:

One of the products of the script/generate scaffold command is a database migration. Migrations are Ruby classes that are designed to make it simple to create and modify database tables. Rails uses rake commands to run migrations, and it's possible to undo a migration after it's been applied to your database. Migration filenames include a timestamp to ensure that they're processed in the order that they were created.

If you look in the db/migrate/20081013124235_create_posts.rb file (remember, yours will have a slightly different name), here's what you'll find:

If you were to translate that into words, it says something like: when this migration is run, create a table named posts with two string columns (name and title) and a text column (content), and generate timestamp fields to track record creation and updating. You can learn the detailed syntax for migrations in the Rails Database Migrations guide.

At this point, you need to do two things: create the database and run the migration. You can use rake commands at the terminal for both of those tasks:

To hook the posts up to the home page you've already created, you can add a link to the home page. Open /app/views/home/index.html.erb and modify it as follows:

The link_to method is one of Rails' built-in view helpers. It creates a hyperlink based on text to display and where to go - in this case, to the path for posts.

Now you're ready to start working with posts. To do that, navigate to http://localhost:3000 and then click the "My Blog" link:

This is the result of Rails rendering the index view of your posts. There aren't currently any posts in the database, but if you click the New Post link you can create one. After that, you'll find that you can edit posts, look at their details, or destroy them. All of the logic and HTML to handle this was built by the single script/generate scaffold command.

Congratulations, you're riding the rails! Now it's time to see how it all works.

The model file, app/models/post.rb is about as simple as it can get:

There isn't much to this file - but note that the Post class inherits from ActiveRecord::Base. Active Record supplies a great deal of functionality to your Rails models for free, including basic database CRUD (Create, Read, Update, Destroy) operations, data validation, as well as sophisticated search support and the ability to relate multiple models to one another.

Rails includes methods to help you validate the data that you send to models. Open the app/models/post.rb file and edit it:

These changes will ensure that all posts have a name and a title, and that the title is at least five characters long. Rails can validate a variety of conditions in a model, including the presence or uniqueness of columns, their format, and the existence of associated objects.

To see your validations in action, you can use the console. The console is a command-line tool that lets you execute Ruby code in the context of your application:

After the console loads, you can use it to work with your application's models:

This code shows creating a new Post instance, attempting to save it and getting false for a return value (indicating that the save failed), and inspecting the errors of the post.

The easiest place to start looking at functionality is with the code that lists all posts. Open the file app/controllers/posts_controller.rb and look at the index action:

This code sets the @posts instance variable to an array of all posts in the database. Post.find(:all) or Post.all calls the Post model to return all of the posts that are currently in the database, with no limiting conditions.

The respond_to block handles both HTML and XML calls to this action. If you browse to http://localhost:3000/posts.xml, you'll see all of the posts in XML format. The HTML format looks for a view in app/views/posts/ with a name that corresponds to the action name. Rails makes all of the instance variables from the action available to the view. Here's app/view/posts/index.html.erb:

This view iterates over the contents of the @posts array to display content and links. A few things to note in the view:

The view is only part of the story of how HTML is displayed in your web browser. Rails also has the concept of layouts, which are containers for views. When Rails renders a view to the browser, it does so by putting the view's HTML into a layout's HTML. The script/generate scaffold command automatically created a default layout, app/views/layouts/posts.html.erb, for the posts. Open this layout in your editor and modify the body tag:

Now when you refresh the /posts page, you'll see a gray background to the page. This same gray background will be used throughout all the views for posts.

Creating a new post involves two actions. The first is the new action, which instantiates an empty Post object:

The new.html.erb view displays this empty Post to the user:

The form_for block is used to create an HTML form. Within this block, you have access to methods to build various controls on the form. For example, f.text_field :name tells Rails to create a text input on the form, and to hook it up to the name attribute of the instance being displayed. You can only use these methods with attributes of the model that the form is based on (in this case name, title, and content). Rails uses form_for in preference to having your write raw HTML because the code is more succinct, and because it explicitly ties the form to a particular model instance.

When the user clicks the Create button on this form, the browser will send information back to the create method of the controller (Rails knows to call the create method because the form is sent with an HTTP POST request; that's one of the conventions that I mentioned earlier):

The create action instantiates a new Post object from the data supplied by the user on the form, which Rails makes available in the params hash. After saving the new post, it uses flash[:notice] to create an informational message for the user, and redirects to the show action for the post. If there's any problem, the create action just shows the new view a second time, with any error messages.

Rails provides the flash hash (usually just called the Flash) so that messages can be carried over to another action, providing the user with useful information on the status of their request. In the case of create, the user never actually sees any page rendered during the Post creation process, because it immediately redirects to the new Post as soon Rails saves the record. The Flash carries over a message to the next action, so that when the user is redirected back to the show action, they are presented with a message saying "Post was successfully created."

When you click the show link for a post on the index page, it will bring you to a URL like http://localhost:3000/posts/1. Rails interprets this as a call to the show action for the resource, and passes in 1 as the :id parameter. Here's the show action:

The show action uses Post.find to search for a single record in the database by its id value. After finding the record, Rails displays it by using show.html.erb:

Like creating a new post, editing a post is a two-part process. The first step is a request to edit_post_path(@post) with a particular post. This calls the edit action in the controller:

After finding the requested post, Rails uses the edit.html.erb view to display it:

Submitting the form created by this view will invoke the update action within the controller:

In the update action, Rails first uses the :id parameter passed back from the edit view to locate the database record that's being edited. The update_attributes call then takes the rest of the parameters from the request and applies them to this record. If all goes well, the user is redirected to the post's show view. If there are any problems, it's back to edit to correct them.

Finally, clicking one of the destroy links sends the associated id to the destroy action:

The destroy method of an Active Record model instance removes the corresponding record from the database. After that's done, there isn't any record to display, so Rails redirects the user's browser to the index view for the model.

At this point, it’s worth looking at some of the tools that Rails provides to eliminate duplication in your code. In particular, you can use partials to clean up duplication in views and filters to help with duplication in controllers.

As you saw earlier, the scaffold-generated views for the new and edit actions are largely identical. You can pull the shared code out into a partial template. This requires editing the new and edit views, and adding a new template:

new.html.erb:

edit.html.erb:

_form.html.erb:

Now, when Rails renders the new or edit view, it will insert the _form partial at the indicated point. Note the naming convention for partials: if you refer to a partial named form inside of a view, the corresponding file is _form.html.erb, with a leading underscore.

For more information on partials, refer to the Layouts and Rending in Rails guide.

At this point, if you look at the controller for posts, you’ll see some duplication:

Four instances of the exact same line of code doesn’t seem very DRY. Rails provides filters as a way to address this sort of repeated code. In this case, you can DRY things up by using a before_filter:

Rails runs before filters before any action in the controller. You can use the :only clause to limit a before filter to only certain actions, or an :except clause to specifically skip a before filter for certain actions. Rails also allows you to define after filters that run after processing an action, as well as around filters that surround the processing of actions. Filters can also be defined in external classes to make it easy to share them between controllers.

For more information on filters, see the Action Controller Basics guide.

Now that you've seen what's in a model built with scaffolding, it's time to add a second model to the application. The second model will handle comments on blog posts.

Models in Rails use a singular name, and their corresponding database tables use a plural name. For the model to hold comments, the convention is to use the name Comment. Even if you don't want to use the entire apparatus set up by scaffolding, most Rails developers still use generators to make things like models and controllers. To create the new model, run this command in your terminal:

This command will generate four files:

First, take a look at comment.rb:

This is very similar to the post.rb model that you saw earlier. The difference is the line belongs_to :post, which sets up an Active Record association. You'll learn a little about associations in the next section of this guide.

In addition to the model, Rails has also made a migration to create the corresponding database table:

The t.references line sets up a foreign key column for the association between the two models. Go ahead and run the migration:

Rails is smart enough to only execute the migrations that have not already been run against this particular database.

Active Record associations let you easily declare the relationship between two models. In the case of comments and posts, you could write out the relationships this way:

In fact, this is very close to the syntax that Rails uses to declare this association. You've already seen the line of code inside the Comment model that makes each comment belong to a Post:

You'll need to edit the post.rb file to add the other side of the association:

These two declarations enable a good bit of automatic behavior. For example, if you have an instance variable @post containing a post, you can retrieve all the comments belonging to that post as the array @post.comments.

Routes are entries in the config/routes.rb file that tell Rails how to match incoming HTTP requests to controller actions. Open up that file and find the existing line referring to posts. Then edit it as follows:

This creates comments as a nested resource within posts. This is another part of capturing the hierarchical relationship that exists between posts and comments.

With the model in hand, you can turn your attention to creating a matching controller. Again, there's a generator for this:

This creates seven files:

The controller will be generated with empty methods for each action that you specified in the call to script/generate controller:

You'll need to flesh this out with code to actually process requests appropriately in each method. Here's a version that (for simplicity's sake) only responds to requests that require HTML:

You'll see a bit more complexity here than you did in the controller for posts. That's a side-effect of the nesting that you've set up; each request for a comment has to keep track of the post to which the comment is attached.

In addition, the code takes advantage of some of the methods available for an association. For example, in the new method, it calls

This creates a new Comment object and sets up the post_id field to have the id from the specified Post object in a single operation.

Because you skipped scaffolding, you'll need to build views for comments "by hand." Invoking script/generate controller will give you skeleton views, but they'll be devoid of actual content. Here's a first pass at fleshing out the comment views.

The index.html.erb view:

The new.html.erb view:

The show.html.erb view:

The edit.html.erb view:

Again, the added complexity here (compared to the views you saw for managing comments) comes from the necessity of juggling a post and its comments at the same time.

As a final step, I'll modify the show.html.erb view for a post to show the comments on that post, and to allow managing those comments:

Note that each post has its own individual comments collection, accessible as @post.comments. That's a consequence of the declarative associations in the models. Path helpers such as post_comments_path come from the nested route declaration in config/routes.rb.

Now that you've seen your first Rails application, you should feel free to update it and experiment on your own. But you don't have to do everything without help. As you need assistance getting up and running with Rails, feel free to consult these support resources:

Rails also comes with built-in help that you can generate using the rake command-line utility:

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