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whether or not a user is logged in. We can use this to show or hide administrative controls (such as adding or editing a given event). Having current_user around will also prove useful in templates, allowing us to access information about the users such as their login name or email address. Rails provides a handy way to extend the visibility of methods to templates by declaring them as helpers. You can use helper_method followed by a symbolic reference to the method in question, as we ve done here. You can also pass an array of method references to helper_method if you want to declare them all at once. Let s apply the filter to our events controller now. We ll also apply a filter to the users controller to restrict who can edit user profiles.

The variety of relationships possible are documented in the official Ruby on Rails documentation at http://api.rubyonrails.org/classes/ActiveRecord/Associations/ ClassMethods.html.

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A useful feature provided by Rails applications out of the box is support for sessions. When a web browser makes a request to your application, Rails silently sends back a cookie containing a unique identifier for that browser. Whenever that browser makes further requests, it sends back the cookie with the unique identifier, so the application always knows when a certain previous visitor is making another request. You can use the session s ability to store information that s specific to a particular visitor for use on future requests. Sessions are commonly used on web sites for features such as shopping carts or keeping track of what pages you ve visited. For example, if you add an item to your cart at an e-commerce site, the item chosen is stored in a data store associated with your session s ID. When you come to check out, your session ID is used to look up data specific to your session in the session system s data store and find out what you have in your cart. To demonstrate basic session storage in your Rails application, you ll count and show a user how many times he or she has accessed actions within your application. To do this, you need to have some way of performing this logic on each request made to the application. You could add logic to every controller action, but an easier way is to use a filter method called before_filter. before_filter is a method you can use at the controller class level to define that a method (or, indeed, many methods) should be executed before the method for the controller action of the current request. Filters make it possible to perform generic activities before every request (or before requests to certain groups of methods or certain controllers).

You apply filters using a declarative syntax. In this case, we want to check that a user is authenticated before we process a protected action, so we ll use the before_filter. Add the filter to the events controller, just inside the class body, as shown in Listing 6-22.

The validates method accepts some other options too; Table 5-9 summarizes the ones that haven t been covered. Table 5-9. Other Options for the validates Method

Note A common use for filters within Rails is to make sure visitors are authenticated and authorized to

class EventsController < ApplicationController before_filter :authenticate, :except => [:index, :show] #... end

visit certain controllers and perform certain actions. If you have a controller class called AdminController, you might want to add a before_filter that ensures a visitor is logged in to the site as an admin user before you let him or her use the potentially dangerous actions within!

Validates that the field is a numeric value Validates that the field value is inside the specified range Validates that the field value is out of the specified range Validates the acceptance of a boolean field