Action Controller is the C in MVC. After routing has determined which controller to use for a request, the controller is responsible for making sense of the request and producing the appropriate output. Luckily, Action Controller does most of the groundwork for you and uses smart conventions to make this as straightforward as possible.
For most conventional [RESTful](http://en.wikipedia.org/wiki/Representational_state_transfer) applications, the controller will receive the request (this is invisible to you as the developer), fetch or save data from a model and use a view to create HTML output. If your controller needs to do things a little differently, that's not a problem, this is just the most common way for a controller to work.
A controller can thus be thought of as a middleman between models and views. It makes the model data available to the view so it can display that data to the user, and it saves or updates user data to the model.
The naming convention of controllers in Rails favors pluralization of the last word in the controller's name, although it is not strictly required (e.g. `ApplicationController`). For example, `ClientsController` is preferable to `ClientController`, `SiteAdminsController` is preferable to `SiteAdminController` or `SitesAdminsController`, and so on.
Following this convention will allow you to use the default route generators (e.g. `resources`, etc) without needing to qualify each `:path` or `:controller`, and will keep URL and path helpers' usage consistent throughout your application. See [Layouts & Rendering Guide](layouts_and_rendering.html) for more details.
A controller is a Ruby class which inherits from `ApplicationController` and has methods just like any other class. When your application receives a request, the routing will determine which controller and action to run, then Rails creates an instance of that controller and runs the method with the same name as the action.
As an example, if a user goes to `/clients/new` in your application to add a new client, Rails will create an instance of `ClientsController` and call its `new` method. Note that the empty method from the example above would work just fine because Rails will by default render the `new.html.erb` view unless the action says otherwise. The `new` method could make available to the view a `@client` instance variable by creating a new `Client`:
The [Layouts & Rendering Guide](layouts_and_rendering.html) explains this in more detail.
`ApplicationController` inherits from `ActionController::Base`, which defines a number of helpful methods. This guide will cover some of these, but if you're curious to see what's in there, you can see all of them in the API documentation or in the source itself.
Only public methods are callable as actions. It is a best practice to lower the visibility of methods (with `private` or `protected`) which are not intended to be actions, like auxiliary methods or filters.
You will probably want to access data sent in by the user or other parameters in your controller actions. There are two kinds of parameters possible in a web application. The first are parameters that are sent as part of the URL, called query string parameters. The query string is everything after "?" in the URL. The second type of parameter is usually referred to as POST data. This information usually comes from an HTML form which has been filled in by the user. It's called POST data because it can only be sent as part of an HTTP POST request. Rails does not make any distinction between query string parameters and POST parameters, and both are available in the `params` hash in your controller:
The `params` hash is not limited to one-dimensional keys and values. It can contain nested arrays and hashes. To send an array of values, append an empty pair of square brackets "[]" to the key name:
NOTE: The actual URL in this example will be encoded as "/clients?ids%5b%5d=1&ids%5b%5d=2&ids%5b%5d=3" as the "[" and "]" characters are not allowed in URLs. Most of the time you don't have to worry about this because the browser will encode it for you, and Rails will decode it automatically, but if you ever find yourself having to send those requests to the server manually you should keep this in mind.
The value of `params[:ids]` will now be `["1", "2", "3"]`. Note that parameter values are always strings; Rails makes no attempt to guess or cast the type.
When this form is submitted, the value of `params[:client]` will be `{ "name" => "Acme", "phone" => "12345", "address" => { "postcode" => "12345", "city" => "Carrot City" } }`. Note the nested hash in `params[:client][:address]`.
If you're writing a web service application, you might find yourself more comfortable accepting parameters in JSON format. If the "Content-Type" header of your request is set to "application/json", Rails will automatically load your parameters into the `params` hash, which you can access as you would normally.
Also, if you've turned on `config.wrap_parameters` in your initializer or called `wrap_parameters` in your controller, you can safely omit the root element in the JSON parameter. In this case, the parameters will be cloned and wrapped with a key chosen based on your controller's name. So the above JSON POST can be written as:
You can customize the name of the key or specific parameters you want to wrap by consulting the [API documentation](http://api.rubyonrails.org/classes/ActionController/ParamsWrapper.html)
The `params` hash will always contain the `:controller` and `:action` keys, but you should use the methods `controller_name` and `action_name` instead to access these values. Any other parameters defined by the routing, such as `:id`, will also be available. As an example, consider a listing of clients where the list can show either active or inactive clients. We can add a route which captures the `:status` parameter in a "pretty" URL:
In this case, when a user opens the URL `/clients/active`, `params[:status]` will be set to "active". When this route is used, `params[:foo]` will also be set to "bar", as if it were passed in the query string. Your controller will also receive `params[:action]` as "index" and `params[:controller]` as "clients".
You can set global default parameters for URL generation by defining a method called `default_url_options` in your controller. Such a method must return a hash with the desired defaults, whose keys must be symbols:
```ruby
class ApplicationController <ActionController::Base
If you define `default_url_options` in `ApplicationController`, as in the example above, these defaults will be used for all URL generation. The method can also be defined in a specific controller, in which case it only affects URLs generated there.
In a given request, the method is not actually called for every single generated URL; for performance reasons, the returned hash is cached, there is at most one invocation per request.
Your application has a session for each user in which you can store small amounts of data that will be persisted between requests. The session is only available in the controller and the view and can use one of a number of different storage mechanisms:
*`ActionDispatch::Session::CookieStore` - Stores everything on the client.
*`ActionDispatch::Session::CacheStore` - Stores the data in the Rails cache.
*`ActionDispatch::Session::ActiveRecordStore` - Stores the data in a database using Active Record. (require `activerecord-session_store` gem).
*`ActionDispatch::Session::MemCacheStore` - Stores the data in a memcached cluster (this is a legacy implementation; consider using CacheStore instead).
All session stores use a cookie to store a unique ID for each session (you must use a cookie, Rails will not allow you to pass the session ID in the URL as this is less secure).
For most stores, this ID is used to look up the session data on the server, e.g. in a database table. There is one exception, and that is the default and recommended session store - the CookieStore - which stores all session data in the cookie itself (the ID is still available to you if you need it). This has the advantage of being very lightweight and it requires zero setup in a new application in order to use the session. The cookie data is cryptographically signed to make it tamper-proof. And it is also encrypted so anyone with access to it can't read its contents. (Rails will not accept it if it has been edited).
The CookieStore can store around 4kB of data - much less than the others - but this is usually enough. Storing large amounts of data in the session is discouraged no matter which session store your application uses. You should especially avoid storing complex objects (anything other than basic Ruby objects, the most common example being model instances) in the session, as the server might not be able to reassemble them between requests, which will result in an error.
If your user sessions don't store critical data or don't need to be around for long periods (for instance if you just use the flash for messaging), you can consider using `ActionDispatch::Session::CacheStore`. This will store sessions using the cache implementation you have configured for your application. The advantage of this is that you can use your existing cache infrastructure for storing sessions without requiring any additional setup or administration. The downside, of course, is that the sessions will be ephemeral and could disappear at any time.
Rails sets up a session key (the name of the cookie) when signing the session data. These can also be changed in `config/initializers/session_store.rb`:
```ruby
# Be sure to restart your server when you modify this file.
NOTE: Changing the secret when using the `CookieStore` will invalidate all existing sessions.
### Accessing the Session
In your controller you can access the session through the `session` instance method.
NOTE: Sessions are lazily loaded. If you don't access sessions in your action's code, they will not be loaded. Hence you will never need to disable sessions, just not accessing them will do the job.
Session values are stored using key/value pairs like a hash:
```ruby
class ApplicationController <ActionController::Base
private
# Finds the User with the ID stored in the session with the key
# :current_user_id This is a common way to handle user login in
# a Rails application; logging in sets the session value and
To store something in the session, just assign it to the key like a hash:
```ruby
class LoginsController <ApplicationController
# "Create" a login, aka "log the user in"
def create
if user = User.authenticate(params[:username], params[:password])
# Save the user ID in the session so it can be used in
# subsequent requests
session[:current_user_id] = user.id
redirect_to root_url
end
end
end
```
To remove something from the session, assign that key to be `nil`:
```ruby
class LoginsController <ApplicationController
# "Delete" a login, aka "log the user out"
def destroy
# Remove the user id from the session
@_current_user = session[:current_user_id] = nil
redirect_to root_url
end
end
```
To reset the entire session, use `reset_session`.
### The Flash
The flash is a special part of the session which is cleared with each request. This means that values stored there will only be available in the next request, which is useful for passing error messages etc.
It is accessed in much the same way as the session, as a hash (it's a [FlashHash](http://api.rubyonrails.org/classes/ActionDispatch/Flash/FlashHash.html) instance).
Let's use the act of logging out as an example. The controller can send a message which will be displayed to the user on the next request:
```ruby
class LoginsController <ApplicationController
def destroy
session[:current_user_id] = nil
flash[:notice] = "You have successfully logged out."
redirect_to root_url
end
end
```
Note that it is also possible to assign a flash message as part of the redirection. You can assign `:notice`, `:alert` or the general purpose `:flash`:
```ruby
redirect_to root_url, notice: "You have successfully logged out."
The `destroy` action redirects to the application's `root_url`, where the message will be displayed. Note that it's entirely up to the next action to decide what, if anything, it will do with what the previous action put in the flash. It's conventional to display any error alerts or notices from the flash in the application's layout:
```erb
<html>
<!-- <head/> -->
<body>
<% flash.each do |name, msg| -%>
<%= content_tag :div, msg, class: name %>
<% end -%>
<!-- more content -->
</body>
</html>
```
This way, if an action sets a notice or an alert message, the layout will display it automatically.
You can pass anything that the session can store; you're not limited to notices and alerts:
```erb
<% if flash[:just_signed_up] %>
<pclass="welcome">Welcome to our site!</p>
<% end %>
```
If you want a flash value to be carried over to another request, use the `keep` method:
```ruby
class MainController <ApplicationController
# Let's say this action corresponds to root_url, but you want
# all requests here to be redirected to UsersController#index.
# If an action sets the flash and redirects here, the values
# would normally be lost when another redirect happens, but you
# can use 'keep' to make it persist for another request.
def index
# Will persist all flash values.
flash.keep
# You can also use a key to keep only some kind of value.
# flash.keep(:notice)
redirect_to users_url
end
end
```
#### `flash.now`
By default, adding values to the flash will make them available to the next request, but sometimes you may want to access those values in the same request. For example, if the `create` action fails to save a resource and you render the `new` template directly, that's not going to result in a new request, but you may still want to display a message using the flash. To do this, you can use `flash.now` in the same way you use the normal `flash`:
Your application can store small amounts of data on the client - called cookies - that will be persisted across requests and even sessions. Rails provides easy access to cookies via the `cookies` method, which - much like the `session` - works like a hash:
ActionController makes it extremely easy to render `XML` or `JSON` data. If you've generated a controller using scaffolding, it would look something like this:
You may notice in the above code that we're using `render xml: @users`, not `render xml: @users.to_xml`. If the object is not a String, then Rails will automatically invoke `to_xml` for us.
"before" filters may halt the request cycle. A common "before" filter is one which requires that a user is logged in for an action to be run. You can define the filter method this way:
The method simply stores an error message in the flash and redirects to the login form if the user is not logged in. If a "before" filter renders or redirects, the action will not run. If there are additional filters scheduled to run after that filter, they are also cancelled.
In this example the filter is added to `ApplicationController` and thus all controllers in the application inherit it. This will make everything in the application require the user to be logged in in order to use it. For obvious reasons (the user wouldn't be able to log in in the first place!), not all controllers or actions should require this. You can prevent this filter from running before particular actions with `skip_before_action`:
Now, the `LoginsController`'s `new` and `create` actions will work as before without requiring the user to be logged in. The `:only` option is used to only skip this filter for these actions, and there is also an `:except` option which works the other way. These options can be used when adding filters too, so you can add a filter which only runs for selected actions in the first place.
"after" filters are similar to "before" filters, but because the action has already been run they have access to the response data that's about to be sent to the client. Obviously, "after" filters cannot stop the action from running.
For example, in a website where changes have an approval workflow an administrator could be able to preview them easily, just apply them within a transaction:
Note that an "around" filter also wraps rendering. In particular, if in the example above, the view itself reads from the database (e.g. via a scope), it will do so within the transaction and thus present the data to preview.
The first is to use a block directly with the *\_action methods. The block receives the controller as an argument. The `require_login` filter from above could be rewritten to use a block:
Note that the filter in this case uses `send` because the `logged_in?` method is private and the filter does not run in the scope of the controller. This is not the recommended way to implement this particular filter, but in more simple cases it might be useful.
The second way is to use a class (actually, any object that responds to the right methods will do) to handle the filtering. This is useful in cases that are more complex and cannot be implemented in a readable and reusable way using the two other methods. As an example, you could rewrite the login filter again to use a class:
Again, this is not an ideal example for this filter, because it's not run in the scope of the controller but gets the controller passed as an argument. The filter class must implement a method with the same name as the filter, so for the `before_action` filter the class must implement a `before` method, and so on. The `around` method must `yield` to execute the action.
Cross-site request forgery is a type of attack in which a site tricks a user into making requests on another site, possibly adding, modifying or deleting data on that site without the user's knowledge or permission.
The first step to avoid this is to make sure all "destructive" actions (create, update and destroy) can only be accessed with non-GET requests. If you're following RESTful conventions you're already doing this. However, a malicious site can still send a non-GET request to your site quite easily, and that's where the request forgery protection comes in. As the name says, it protects from forged requests.
The way this is done is to add a non-guessable token which is only known to your server to each request. This way, if a request comes in without the proper token, it will be denied access.
If you generate a form like this:
```erb
<%= form_for @user do |f| %>
<%= f.text_field :username %>
<%= f.text_field :password %>
<% end %>
```
You will see how the token gets added as a hidden field:
Rails adds this token to every form that's generated using the [form helpers](form_helpers.html), so most of the time you don't have to worry about it. If you're writing a form manually or need to add the token for another reason, it's available through the method `form_authenticity_token`:
The `form_authenticity_token` generates a valid authentication token. That's useful in places where Rails does not add it automatically, like in custom Ajax calls.
The [Security Guide](security.html) has more about this and a lot of other security-related issues that you should be aware of when developing a web application.
The Request and Response Objects
--------------------------------
In every controller there are two accessor methods pointing to the request and the response objects associated with the request cycle that is currently in execution. The `request` method contains an instance of `AbstractRequest` and the `response` method returns a response object representing what is going to be sent back to the client.
### The `request` Object
The request object contains a lot of useful information about the request coming in from the client. To get a full list of the available methods, refer to the [API documentation](http://api.rubyonrails.org/classes/ActionDispatch/Request.html). Among the properties that you can access on this object are:
| domain(n=2) | The hostname's first `n` segments, starting from the right (the TLD). |
| format | The content type requested by the client. |
| method | The HTTP method used for the request. |
| get?, post?, patch?, put?, delete?, head? | Returns true if the HTTP method is GET/POST/PATCH/PUT/DELETE/HEAD. |
| headers | Returns a hash containing the headers associated with the request. |
| port | The port number (integer) used for the request. |
| protocol | Returns a string containing the protocol used plus "://", for example "http://". |
| query_string | The query string part of the URL, i.e., everything after "?". |
| remote_ip | The IP address of the client. |
| url | The entire URL used for the request. |
#### `path_parameters`, `query_parameters`, and `request_parameters`
Rails collects all of the parameters sent along with the request in the `params` hash, whether they are sent as part of the query string or the post body. The request object has three accessors that give you access to these parameters depending on where they came from. The `query_parameters` hash contains parameters that were sent as part of the query string while the `request_parameters` hash contains parameters sent as part of the post body. The `path_parameters` hash contains parameters that were recognized by the routing as being part of the path leading to this particular controller and action.
### The `response` Object
The response object is not usually used directly, but is built up during the execution of the action and rendering of the data that is being sent back to the user, but sometimes - like in an after filter - it can be useful to access the response directly. Some of these accessor methods also have setters, allowing you to change their values.
| body | This is the string of data being sent back to the client. This is most often HTML. |
| status | The HTTP status code for the response, like 200 for a successful request or 404 for file not found. |
| location | The URL the client is being redirected to, if any. |
| content_type | The content type of the response. |
| charset | The character set being used for the response. Default is "utf-8". |
| headers | Headers used for the response. |
#### Setting Custom Headers
If you want to set custom headers for a response then `response.headers` is the place to do it. The headers attribute is a hash which maps header names to their values, and Rails will set some of them automatically. If you want to add or change a header, just assign it to `response.headers` this way:
Rails comes with two built-in HTTP authentication mechanisms:
* Basic Authentication
* Digest Authentication
### HTTP Basic Authentication
HTTP basic authentication is an authentication scheme that is supported by the majority of browsers and other HTTP clients. As an example, consider an administration section which will only be available by entering a username and a password into the browser's HTTP basic dialog window. Using the built-in authentication is quite easy and only requires you to use one method, `http_basic_authenticate_with`.
With this in place, you can create namespaced controllers that inherit from `AdminsController`. The filter will thus be run for all actions in those controllers, protecting them with HTTP basic authentication.
HTTP digest authentication is superior to the basic authentication as it does not require the client to send an unencrypted password over the network (though HTTP basic authentication is safe over HTTPS). Using digest authentication with Rails is quite easy and only requires using one method, `authenticate_or_request_with_http_digest`.
As seen in the example above, the `authenticate_or_request_with_http_digest` block takes only one argument - the username. And the block returns the password. Returning `false` or `nil` from the `authenticate_or_request_with_http_digest` will cause authentication failure.
Streaming and File Downloads
----------------------------
Sometimes you may want to send a file to the user instead of rendering an HTML page. All controllers in Rails have the `send_data` and the `send_file` methods, which will both stream data to the client. `send_file` is a convenience method that lets you provide the name of a file on the disk and it will stream the contents of that file for you.
To stream data to the client, use `send_data`:
```ruby
require "prawn"
class ClientsController <ApplicationController
# Generates a PDF document with information on the client and
# returns it. The user will get the PDF as a file download.
The `download_pdf` action in the example above will call a private method which actually generates the PDF document and returns it as a string. This string will then be streamed to the client as a file download and a filename will be suggested to the user. Sometimes when streaming files to the user, you may not want them to download the file. Take images, for example, which can be embedded into HTML pages. To tell the browser a file is not meant to be downloaded, you can set the `:disposition` option to "inline". The opposite and default value for this option is "attachment".
### Sending Files
If you want to send a file that already exists on disk, use the `send_file` method.
```ruby
class ClientsController <ApplicationController
# Stream a file that has already been generated and stored on disk.
This will read and stream the file 4kB at the time, avoiding loading the entire file into memory at once. You can turn off streaming with the `:stream` option or adjust the block size with the `:buffer_size` option.
If `:type` is not specified, it will be guessed from the file extension specified in `:filename`. If the content type is not registered for the extension, `application/octet-stream` will be used.
WARNING: Be careful when using data coming from the client (params, cookies, etc.) to locate the file on disk, as this is a security risk that might allow someone to gain access to files they are not meant to.
TIP: It is not recommended that you stream static files through Rails if you can instead keep them in a public folder on your web server. It is much more efficient to let the user download the file directly using Apache or another web server, keeping the request from unnecessarily going through the whole Rails stack.
### RESTful Downloads
While `send_data` works just fine, if you are creating a RESTful application having separate actions for file downloads is usually not necessary. In REST terminology, the PDF file from the example above can be considered just another representation of the client resource. Rails provides an easy and quite sleek way of doing "RESTful downloads". Here's how you can rewrite the example so that the PDF download is a part of the `show` action, without any streaming:
```ruby
class ClientsController <ApplicationController
# The user can request to receive this resource as HTML or PDF.
def show
@client = Client.find(params[:id])
respond_to do |format|
format.html
format.pdf { render pdf: generate_pdf(@client) }
end
end
end
```
In order for this example to work, you have to add the PDF MIME type to Rails. This can be done by adding the following line to the file `config/initializers/mime_types.rb`:
```ruby
Mime::Type.register "application/pdf", :pdf
```
NOTE: Configuration files are not reloaded on each request, so you have to restart the server in order for their changes to take effect.
Now the user can request to get a PDF version of a client just by adding ".pdf" to the URL:
Rails keeps a log file for each environment in the `log` folder. These are extremely useful when debugging what's actually going on in your application, but in a live application you may not want every bit of information to be stored in the log file.
You can filter out sensitive request parameters from your log files by appending them to `config.filter_parameters` in the application configuration. These parameters will be marked [FILTERED] in the log.
Most likely your application is going to contain bugs or otherwise throw an exception that needs to be handled. For example, if the user follows a link to a resource that no longer exists in the database, Active Record will throw the `ActiveRecord::RecordNotFound` exception.
Rails' default exception handling displays a "500 Server Error" message for all exceptions. If the request was made locally, a nice traceback and some added information gets displayed so you can figure out what went wrong and deal with it. If the request was remote Rails will just display a simple "500 Server Error" message to the user, or a "404 Not Found" if there was a routing error or a record could not be found. Sometimes you might want to customize how these errors are caught and how they're displayed to the user. There are several levels of exception handling available in a Rails application:
### The Default 500 and 404 Templates
By default a production application will render either a 404 or a 500 error message. These messages are contained in static HTML files in the `public` folder, in `404.html` and `500.html` respectively. You can customize these files to add some extra information and layout, but remember that they are static; i.e. you can't use RHTML or layouts in them, just plain HTML.
### `rescue_from`
If you want to do something a bit more elaborate when catching errors, you can use `rescue_from`, which handles exceptions of a certain type (or multiple types) in an entire controller and its subclasses.
When an exception occurs which is caught by a `rescue_from` directive, the exception object is passed to the handler. The handler can be a method or a `Proc` object passed to the `:with` option. You can also use a block directly instead of an explicit `Proc` object.
Here's how you can use `rescue_from` to intercept all `ActiveRecord::RecordNotFound` errors and do something with them.
```ruby
class ApplicationController <ActionController::Base
Of course, this example is anything but elaborate and doesn't improve on the default exception handling at all, but once you can catch all those exceptions you're free to do whatever you want with them. For example, you could create custom exception classes that will be thrown when a user doesn't have access to a certain section of your application:
```ruby
class ApplicationController <ActionController::Base
WARNING: You shouldn't do `rescue_from Exception` or `rescue_from StandardError` unless you have a particular reason as it will cause serious side-effects (e.g. you won't be able to see exception details and tracebacks during development).
NOTE: Certain exceptions are only rescuable from the `ApplicationController` class, as they are raised before the controller gets initialized and the action gets executed. See Pratik Naik's [article](http://m.onkey.org/2008/7/20/rescue-from-dispatching) on the subject for more information.
Sometime you might want to force a particular controller to only be accessible via an HTTPS protocol for security reasons. You can use the `force_ssl` method in your controller to enforce that:
Please note that if you find yourself adding `force_ssl` to many controllers, you may want to force the whole application to use HTTPS instead. In that case, you can set the `config.force_ssl` in your environment file.
