From 37ea9de5af3fc8c133bbff060c11750bcb6970de Mon Sep 17 00:00:00 2001 From: Mike Gunderloy Date: Tue, 4 Nov 2008 20:18:47 -0600 Subject: Copy edit on AC Basics Guide --- .../doc/guides/html/actioncontroller_basics.html | 296 +++++++++++++++------ 1 file changed, 217 insertions(+), 79 deletions(-) (limited to 'railties/doc/guides/html/actioncontroller_basics.html') diff --git a/railties/doc/guides/html/actioncontroller_basics.html b/railties/doc/guides/html/actioncontroller_basics.html index 23d3c2e450..d58536cc37 100644 --- a/railties/doc/guides/html/actioncontroller_basics.html +++ b/railties/doc/guides/html/actioncontroller_basics.html @@ -199,18 +199,20 @@ ul#navMain {

Chapters

  1. - What does a controller do? + What Does a Controller do?
  2. - Methods and actions + Methods and Actions
  3. Parameters
    4. @@ -218,9 +220,9 @@ ul#navMain { Session
@@ -285,24 +293,69 @@ ul#navMain {

Action Controller basics

-

In this guide you will learn how controllers work and how they fit into the request cycle in your application. You will learn how to make use of the many tools provided by Action Controller to work with the session, cookies and filters and how to use the built-in HTTP authentication and data streaming facilities. In the end, we will take a look at some tools that will be useful once your controllers are ready and working, like how to filter sensitive parameters from the log and how to rescue and deal with exceptions that may be raised during the request.

+

In this guide you will learn how controllers work and how they fit into the request cycle in your application. After reading this guide, you will be able to:

+
    +
  • +

    +Follow the flow of a request through a controller +

    +
    • +
  • +

    +Understand why and how to store data in the session or cookies +

    +
    • +
  • +

    +Work with filters to execute code during request processing +

    +
    • +
  • +

    +Use Action Controller's built-in HTTP authentication +

    +
    • +
  • +

    +Stream data directly to the user's browser +

    +
    • +
  • +

    +Filter sensitive parameters so they do not appear in the application's log +

    +
    • +
  • +

    +Deal with exceptions that may be raised during request processing +

    +
    • +
-

1. What does a controller do?

+

1. What Does a Controller do?

Action Controller is the C in MVC. After routing has determined which controller to use for a request, your 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 straight-forward as possible.

For most conventional RESTful 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 middle man between models and views. It makes the model data available to the view so it can display it to the user, and it saves or updates data from the user to the model.

+

A controller can thus be thought of as a middle man 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 data from the user to the model.

+
+ + + +
+Note +For more details on the routing process, see Rails Routing from the Outside In.
+
-

2. Methods and actions

+

2. Methods and Actions

-

A controller is a Ruby class which inherits from ActionController::Base and has methods just like any other class. Usually these methods correspond to actions in MVC, but they can just as well be helpful methods which can be called by actions. When your application receives a request, the routing will determine which controller and action to run. Then an instance of that controller will be created and the method corresponding to the action (the method with the same name as the action) gets run.

+

A controller is a Ruby class which inherits from ApplicationController and has methods just like any other class. Usually these methods correspond to actions in MVC, but they can just as well be helpful methods which can be called by actions. 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 corresponding to the action (the method with the same name as the action).

-
class ClientsController < ActionController::Base
+
class ClientsController < ApplicationController
 
   # Actions are public methods
   def new
@@ -321,7 +374,7 @@ private
 end

Private methods in a controller are also used as filters, which will be covered later in this guide.

-

As an example, if the user goes to /clients/new in your application to add a new client, a ClientsController instance will be created and the new method will be run. Note that the empty method from the example above could 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:

+

As an example, if the user goes to /clients/new in your application to add a new client, Rails will create a ClientsController instance will be created and run the new method. Note that the empty method from the example above could 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:

end

The Layouts & rendering guide 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.

3. Parameters

-

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, 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 a 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:

+

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 a 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:

class ClientsController < ActionController::Base
 
-  # This action uses query string parameters because it gets run by a HTTP GET request,
-  # but this does not make any difference to the way in which the parameters are accessed.
-  # The URL for this action would look like this in order to list activated clients: /clients?status=activated
+  # This action uses query string parameters because it gets run by a HTTP
+  # GET request, but this does not make any difference to the way in which
+  # the parameters are accessed. The URL for this action would look like this
+  # in order to list activated clients: /clients?status=activated
   def index
     if params[:status] = "activated"
       @clients = Client.activated
@@ -370,11 +425,11 @@ http://www.gnu.org/software/src-highlite -->
 
 end
-

3.1. Hash and array parameters

+

3.1. Hash and Array Parameters

The params hash is not limited to one-dimensional keys and values. It can contain arrays and (nested) hashes. To send an array of values, append "[]" to the key name:

-
GET /clients?ids[]=1&ids[2]&ids[]=3
+
GET /clients?ids[]=1&ids[]=2&ids[]=3

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.

To send a hash you include the key name inside the brackets:

@@ -388,8 +443,32 @@ http://www.gnu.org/software/src-highlite --> </form>

The value of params[:client] when this form is submitted will be {:name ⇒ "Acme", :phone ⇒ "12345", :address ⇒ {:postcode ⇒ "12345", :city ⇒ "Carrot City"}}. Note the nested hash in params[:client][:address].

-

3.2. Routing parameters

-

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.

+

3.2. Routing Parameters

+

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:

+
+
+
# ...
+map.connect "/clients/:status", :controller => "clients", :action => "index", :foo => "bar"
+# ...
+
+

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" just like it was passed in the query string in the same way params[:action] will contain "index".

+

3.3. default_url_options

+

You can set global default parameters that will be used when generating URLs with default_url_options. To do this, define a method with that name in your controller:

+
+
+
class ApplicationController < ActionController::Base
+
+  #The options parameter is the hash passed in to url_for
+  def default_url_options(options)
+    {:locale => I18n.locale}
+  end
+
+end
+
+

These options will be used as a starting-point when generating, so it's possible they'll be overridden by url_for. Because this method is defined in the controller, you can define it on ApplicationController so it would be used for all URL generation, or you could define it on only one controller for all URLs generated there.

4. Session

@@ -416,8 +495,9 @@ ActiveRecordStore - Stores the data in a database using Active Record.

-

All session stores store the session id in a cookie - there is no other way of passing it to the server. Most stores also use this key to locate the session data on the server.

-

The default and recommended store, the Cookie Store, does not store session data on the server, but in the cookie itself. The data is cryptographically signed to make it tamper-proof, but it is not encrypted, so anyone with access to it can read its contents. It can only store about 4kB of data - much less than the others - but this is usually enough. Storing large amounts of data is discouraged no matter which session store your application uses. Expecially discouraged is storing complex objects (anything other than basic Ruby objects, the primary 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. The Cookie Store has the added advantage that it does not require any setting up beforehand - Rails will generate a "secret key" which will be used to sign the cookie when you create the application.

+

All session stores store either the session ID or the entire session in a cookie - Rails does not allow the session ID to be passed in any other way. Most stores also use this key to locate the session data on the server.

+

The default and recommended store, the Cookie Store, does not store session data on the server, but in the cookie itself. The data is cryptographically signed to make it tamper-proof, but it is not encrypted, so anyone with access to it can read its contents but not edit it. It can only store about 4kB of data - much less than the others - but this is usually enough. Storing large amounts of data is discouraged no matter which session store your application uses. You should especially avoid storing complex objects (anything other than basic Ruby objects, the primary 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. The Cookie Store has the added advantage that it does not require any setting up beforehand - Rails will generate a "secret key" which will be used to sign the cookie when you create the application.

+

Read more about session storage in the Security Guide.

If you need a different session storage mechanism, you can change it in the config/environment.rb file:

# Set to one of [:active_record_store, :drb_store, :mem_cache_store, :cookie_store]
 config.action_controller.session_store = :active_record_store
-

4.1. Disabling the session

-

Sometimes you don't need a session, and you can turn it off to avoid the unnecessary overhead. To do this, use the session class method in your controller:

+

4.1. Disabling the Session

+

Sometimes you don't need a session. In this case, you can turn it off to avoid the unnecessary overhead. To do this, use the session class method in your controller:

session :on end
-

Or even a single action:

+

Or even for specified actions:

session :on, :only => [:create, :update] end
-

4.2. Accessing the session

+

4.2. Accessing the Session

In your controller you can access the session through the session instance method.

@@ -481,7 +561,7 @@ http://www.gnu.org/software/src-highlite --> private # Finds the User with the ID stored in the session with the key :current_user_id - # This is a common way to do user login in a Rails application; logging in sets the + # This is a common way to handle user login in a Rails application; logging in sets the# session value and logging out removes it.def current_user @_current_user||= session[:current_user_id]&& User.find(session[:current_user_id]) @@ -525,9 +605,9 @@ http://www.gnu.org/software/src-highlite --> end -

To reset the entire session, use reset_session.

+

To reset the entire session, use reset_session.

4.3. 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 storing error messages etc. It is accessed in much the same way as the session, like a hash. Let's use the act of logging out as an example. The controller can set a message which will be displayed to the user on the next request:

+

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 storing error messages etc. It is accessed in much the same way as the session, like a hash. 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:

end
-

4.3.1. flash.now

+

4.3.1. 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:

end
-

Note that while for session values, you set the key to nil, to delete a cookie value, you use cookies.delete(:key).

+

Note that while for session values, you set the key to nil, to delete a cookie value, you should use cookies.delete(:key).

6. Filters

-

Filters are methods that are run before, after or "around" a controller action. For example, one filter might check to see if the logged in user has the right credentials to access that particular controller or action. Filters are inherited, so if you set a filter on ApplicationController, it will be run on every controller in your application. A common, simple filter is one which requires that a user is logged in for an action to be run. Let's define the filter method first:

+

Filters are methods that are run before, after or "around" a controller action. For example, one filter might check to see if the logged in user has the right credentials to access that particular controller or action. Filters are inherited, so if you set a filter on ApplicationController, it will be run on every controller in your application. A common, simple 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:

end
-

In this example, the filter is added to ApplicationController and thus all controllers in the application. 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, so to prevent this filter from running you can use skip_before_filter :

+

In this example, the filter is added to ApplicationController and thus all controllers in the application. 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_filter :

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.

-

6.1. After filters and around filters

+

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.

+

6.1. After Filters and Around Filters

In addition to the before filters, you can run filters after an action has run or both before and after. The after filter is similar to the before filter, but because the action has already been run it has access to the response data that's about to be sent to the client. Obviously, after filters can not stop the action from running. Around filters are responsible for running the action, but they can choose not to, which is the around filter's way of stopping it.

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 than can not be implemented in a readable and reusable way using the two other methods. As an example, we will rewrite the login filter again to use a class:

+

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 than can not 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:

end
-

Again, this is not an ideal example for this filter, because it's not run in the scope of the controller but gets it passed as an argument. The filter class has a class method filter which gets run before or after the action, depending on if it's a before or after filter. Classes used as around filters can also use the same filter method, which will get run in the same way. The method must yield to execute the action. Alternatively, it can have both a before and an after method that are run before and after the action.

+

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 has a class method filter which gets run before or after the action, depending on if it's a before or after filter. Classes used as around filters can also use the same filter method, which will get run in the same way. The method must yield to execute the action. Alternatively, it can have both a before and an after method that are run before and after the action.

The Rails API documentation has more information on using filters.

7. Verification

-

Verifications make sure certain criterias are met in order for a controller or action to run. They can specify that a certain key (or several keys in the form of an array) is present in the params, session or flash hashes or that a certain HTTP method was used or that the request was made using XMLHTTPRequest (Ajax). The default action taken when these criterias are not met is to render a 400 Bad Request response, but you can customize this by specifying a redirect URL or rendering something else and you can also add flash messages and HTTP headers to the response. It is described in the API codumentation as "essentially a special kind of before_filter".

-

Let's see how we can use verification to make sure the user supplies a username and a password in order to log in:

+

Verifications make sure certain criteria are met in order for a controller or action to run. They can specify that a certain key (or several keys in the form of an array) is present in the params, session or flash hashes or that a certain HTTP method was used or that the request was made using XMLHTTPRequest (Ajax). The default action taken when these criteria are not met is to render a 400 Bad Request response, but you can customize this by specifying a redirect URL or rendering something else and you can also add flash messages and HTTP headers to the response. It is described in the API documentation as "essentially a special kind of before_filter".

+

Here's an example of using verification to make sure the user supplies a username and a password in order to log in:

verify :params => [:username, :password], :render => {:action => "new"}, :add_flash => {:error => "Username and password required to log in"}, - :only => :create #Only run this verification for the "create" action + :only => :create # Only run this verification for the "create" action end
-

8. The request and response objects

+

8. Request Forgery Protection

-

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 contains the response object representing what is going to be sent back to the client.

-

8.1. The request

-

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.

+

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:

+
+
+
<% 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:

+
+
+
<form action="/users/1" method="post">
+<div><!-- ... --><input type="hidden" value="67250ab105eb5ad10851c00a5621854a23af5489" name="authenticity_token"/></div>
+<!-- Fields -->
+</form>
+
+

Rails adds this token to every form that's generated using the form helpers, 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:

+
+
Example: Add a JavaScript variable containing the token for use with Ajax
+
+
<%= javascript_tag "MyApp.authenticity_token = '#{form_authenticity_token}'" %>
+
+

The Security Guide has more about this and a lot of other security-related issues that you should be aware of when developing a web application.

+
+

9. 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.

+

9.1. 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. Among the properties that you can access on this object:

  • @@ -812,7 +925,7 @@ host - The hostname used for this request.

  • -domain - The hostname without the first part (usually "www"). +domain - The hostname without the first segment (usually "www").

  • @@ -861,9 +974,9 @@ url - The entire URL used for the request.

-

8.1.1. 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 recognised by the routing as being part of the path leading to this particular controller and action.

-

8.2. The response

+

9.1.1. 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.

+

9.2. 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.

  • @@ -891,11 +1004,25 @@ content_type - The content type of the response. charset - The character set being used for the response. Default is "utf8".

    • +
  • +

    +headers - Headers used for the response. +

    +
+

9.2.1. 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 - like "Content-Type" - automatically. If you want to add or change a header, just assign it to headers with the name and value:

+
+
+
response.headers["Content-Type"] = "application/pdf"
+
-

9. HTTP Basic Authentication

+

10. HTTP Basic Authentication

-

Rails comes with built-in HTTP Basic authentication. This is an authentication scheme that is supported by the majority of browsers and other HTTP clients. As an example, we will create 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, authenticate_or_request_with_http_basic.

+

Rails comes with built-in HTTP Basic authentication. This 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, authenticate_or_request_with_http_basic.

- +
Warning Be careful when using (or just don't use) "outside" data (params, cookies, etc) to locate the file on disk, as this is a security risk as someone could gain access to files they are not meant to have access to.Be careful when using (or just don't use) "outside" data (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 see.
@@ -985,8 +1112,8 @@ http://www.gnu.org/software/src-highlite --> 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.
-

10.2. 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". Let's try to rewrite the example so that the PDF download is a part of the show action:

+

11.2. 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:

end
-

In order for this example to work, we 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:

+

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:

GET /clients/1.pdf
-

11. Parameter filtering

+

12. Parameter Filtering

-

Rails keeps a log file for each environment (development, test and production) 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. The filter_parameter_logging method can be used to filter out sensitive information from the log. It works by replacing certain values in the params hash with "[FILTERED]" as they are written to the log. As an example, let's see how to filter all parameters with keys that include "password":

+

Rails keeps a log file for each environment (development, test and production) 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. The filter_parameter_logging method can be used to filter out sensitive information from the log. It works by replacing certain values in the params hash with "[FILTERED]" as they are written to the log. As an example, let's see how to filter all parameters with keys that include "password":

The method works recursively through all levels of the params hash and takes an optional second parameter which is used as the replacement string if present. It can also take a block which receives each key in return and replaces those for which the block returns true.

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12. Rescue

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13. 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:

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12.1. The default 500 and 404 templates

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13.1. 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.

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12.2. rescue_from

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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.

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Let's see how we can use rescue_from to intercept all ActiveRecord::RecordNotFound errors and do something with them.

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13.2. rescue_from

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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.

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Here's how you can use rescue_from to intercept all ActiveRecord::RecordNotFound errors and do something with them.