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<% 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`: ```ruby class ClientsController < ApplicationController def create @client = Client.new(params[:client]) if @client.save # ... else flash.now[:error] = "Could not save client" render action: "new" end end end ``` Cookies ------- 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: ```ruby class CommentsController < ApplicationController def new # Auto-fill the commenter's name if it has been stored in a cookie @comment = Comment.new(author: cookies[:commenter_name]) end def create @comment = Comment.new(params[:comment]) if @comment.save flash[:notice] = "Thanks for your comment!" if params[:remember_name] # Remember the commenter's name. cookies[:commenter_name] = @comment.author else # Delete cookie for the commenter's name cookie, if any. cookies.delete(:commenter_name) end redirect_to @comment.article else render action: "new" end end end ``` Note that while for session values you set the key to `nil`, to delete a cookie value you should use `cookies.delete(:key)`. Rails also provides a signed cookie jar and an encrypted cookie jar for storing sensitive data. The signed cookie jar appends a cryptographic signature on the cookie values to protect their integrity. The encrypted cookie jar encrypts the values in addition to signing them, so that they cannot be read by the end user. Refer to the [API documentation](http://api.rubyonrails.org/classes/ActionDispatch/Cookies.html) for more details. These special cookie jars use a serializer to serialize the assigned values into strings and deserializes them into Ruby objects on read. You can specify what serializer to use: ```ruby Rails.application.config.action_dispatch.cookies_serializer = :json ``` The default serializer for new applications is `:json`. For compatibility with old applications with existing cookies, `:marshal` is used when `serializer` option is not specified. You may also set this option to `:hybrid`, in which case Rails would transparently deserialize existing (`Marshal`-serialized) cookies on read and re-write them in the `JSON` format. This is useful for migrating existing applications to the `:json` serializer. It is also possible to pass a custom serializer that responds to `load` and `dump`: ```ruby Rails.application.config.action_dispatch.cookies_serializer = MyCustomSerializer ``` When using the `:json` or `:hybrid` serializer, you should beware that not all Ruby objects can be serialized as JSON. For example, `Date` and `Time` objects will be serialized as strings, and `Hash`es will have their keys stringified. ```ruby class CookiesController < ApplicationController def set_cookie cookies.encrypted[:expiration_date] = Date.tomorrow # => Thu, 20 Mar 2014 redirect_to action: 'read_cookie' end def read_cookie cookies.encrypted[:expiration_date] # => "2014-03-20" end end ``` It's advisable that you only store simple data (strings and numbers) in cookies. If you have to store complex objects, you would need to handle the conversion manually when reading the values on subsequent requests. If you use the cookie session store, this would apply to the `session` and `flash` hash as well. Rendering XML and JSON data --------------------------- 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: ```ruby class UsersController < ApplicationController def index @users = User.all respond_to do |format| format.html # index.html.erb format.xml { render xml: @users} format.json { render json: @users} end end end ``` 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. Filters ------- Filters are methods that are run before, after or "around" a controller action. Filters are inherited, so if you set a filter on `ApplicationController`, it will be run on every controller in your application. "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: ```ruby class ApplicationController < ActionController::Base before_action :require_login private def require_login unless logged_in? flash[:error] = "You must be logged in to access this section" redirect_to new_login_url # halts request cycle end end end ``` 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`: ```ruby class LoginsController < ApplicationController skip_before_action :require_login, only: [:new, :create] end ``` 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 and Around Filters In addition to "before" filters, you can also run filters after an action has been executed, or both before and after. "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. "Around" filters are responsible for running their associated actions by yielding, similar to how Rack middlewares work. 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: ```ruby class ChangesController < ApplicationController around_action :wrap_in_transaction, only: :show private def wrap_in_transaction ActiveRecord::Base.transaction do begin yield ensure raise ActiveRecord::Rollback end end end end ``` 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. You can choose not to yield and build the response yourself, in which case the action will not be run. ### Other Ways to Use Filters While the most common way to use filters is by creating private methods and using *_action to add them, there are two other ways to do the same thing. The first is to use a block directly with the *\_action methods. The block receives the controller as an argument, and the `require_login` filter from above could be rewritten to use a block: ```ruby class ApplicationController < ActionController::Base before_action do |controller| unless controller.send(:logged_in?) flash[:error] = "You must be logged in to access this section" redirect_to new_login_url end end end ``` Note that the filter in this case uses `send` because the `logged_in?` method is private and the filter is 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: ```ruby class ApplicationController < ActionController::Base before_action LoginFilter end class LoginFilter def self.before(controller) unless controller.send(:logged_in?) controller.flash[:error] = "You must be logged in to access this section" controller.redirect_to controller.new_login_url end end end ``` 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. Request Forgery Protection -------------------------- 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: ```html ``` 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: | Property of `request` | Purpose | | ----------------------------------------- | -------------------------------------------------------------------------------- | | host | The hostname used for this request. | | 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. | Property of `response` | Purpose | | ---------------------- | --------------------------------------------------------------------------------------------------- | | 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: ```ruby response.headers["Content-Type"] = "application/pdf" ``` Note: in the above case it would make more sense to use the `content_type` setter directly. HTTP Authentications -------------------- 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`. ```ruby class AdminsController < ApplicationController http_basic_authenticate_with name: "humbaba", password: "5baa61e4" end ``` 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 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`. ```ruby class AdminsController < ApplicationController USERS = { "lifo" => "world" } before_action :authenticate private def authenticate authenticate_or_request_with_http_digest do |username| USERS[username] end end end ``` 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. def download_pdf client = Client.find(params[:id]) send_data generate_pdf(client), filename: "#{client.name}.pdf", type: "application/pdf" end private def generate_pdf(client) Prawn::Document.new do text client.name, align: :center text "Address: #{client.address}" text "Email: #{client.email}" end.render end end ``` 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. def download_pdf client = Client.find(params[:id]) send_file("#{Rails.root}/files/clients/#{client.id}.pdf", filename: "#{client.name}.pdf", type: "application/pdf") end end ``` 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: ```bash GET /clients/1.pdf ``` ### Live Streaming of Arbitrary Data Rails allows you to stream more than just files. In fact, you can stream anything you would like in a response object. The `ActionController::Live` module allows you to create a persistent connection with a browser. Using this module, you will be able to send arbitrary data to the browser at specific points in time. #### Incorporating Live Streaming Including `ActionController::Live` inside of your controller class will provide all actions inside of the controller the ability to stream data. You can mix in the module like so: ```ruby class MyController < ActionController::Base include ActionController::Live def stream response.headers['Content-Type'] = 'text/event-stream' 100.times { response.stream.write "hello world\n" sleep 1 } ensure response.stream.close end end ``` The above code will keep a persistent connection with the browser and send 100 messages of `"hello world\n"`, each one second apart. There are a couple of things to notice in the above example. We need to make sure to close the response stream. Forgetting to close the stream will leave the socket open forever. We also have to set the content type to `text/event-stream` before we write to the response stream. This is because headers cannot be written after the response has been committed (when `response.committed` returns a truthy value), which occurs when you `write` or `commit` the response stream. #### Example Usage Let's suppose that you were making a Karaoke machine and a user wants to get the lyrics for a particular song. Each `Song` has a particular number of lines and each line takes time `num_beats` to finish singing. If we wanted to return the lyrics in Karaoke fashion (only sending the line when the singer has finished the previous line), then we could use `ActionController::Live` as follows: ```ruby class LyricsController < ActionController::Base include ActionController::Live def show response.headers['Content-Type'] = 'text/event-stream' song = Song.find(params[:id]) song.each do |line| response.stream.write line.lyrics sleep line.num_beats end ensure response.stream.close end end ``` The above code sends the next line only after the singer has completed the previous line. #### Streaming Considerations Streaming arbitrary data is an extremely powerful tool. As shown in the previous examples, you can choose when and what to send across a response stream. However, you should also note the following things: * Each response stream creates a new thread and copies over the thread local variables from the original thread. Having too many thread local variables can negatively impact performance. Similarly, a large number of threads can also hinder performance. * Failing to close the response stream will leave the corresponding socket open forever. Make sure to call `close` whenever you are using a response stream. * WEBrick servers buffer all responses, and so including `ActionController::Live` will not work. You must use a web server which does not automatically buffer responses. Log Filtering ------------- 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. ### Parameters Filtering 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. ```ruby config.filter_parameters << :password ``` ### Redirects Filtering Sometimes it's desirable to filter out from log files some sensitive locations your application is redirecting to. You can do that by using the `config.filter_redirect` configuration option: ```ruby config.filter_redirect << 's3.amazonaws.com' ``` You can set it to a String, a Regexp, or an array of both. ```ruby config.filter_redirect.concat ['s3.amazonaws.com', /private_path/] ``` Matching URLs will be marked as '[FILTERED]'. Rescue ------ 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 rescue_from ActiveRecord::RecordNotFound, with: :record_not_found private def record_not_found render plain: "404 Not Found", status: 404 end end ``` 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 rescue_from User::NotAuthorized, with: :user_not_authorized private def user_not_authorized flash[:error] = "You don't have access to this section." redirect_to :back end end class ClientsController < ApplicationController # Check that the user has the right authorization to access clients. before_action :check_authorization # Note how the actions don't have to worry about all the auth stuff. def edit @client = Client.find(params[:id]) end private # If the user is not authorized, just throw the exception. def check_authorization raise User::NotAuthorized unless current_user.admin? end end ``` 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. Force HTTPS protocol -------------------- 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: ```ruby class DinnerController force_ssl end ``` Just like the filter, you could also pass `:only` and `:except` to enforce the secure connection only to specific actions: ```ruby class DinnerController force_ssl only: :cheeseburger # or force_ssl except: :cheeseburger end ``` 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.