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Automatic Local Variable Assignment in Partials
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Partials are distinct by virtue of a convenience convention: the value of an instance variable with the same name as the partial will be available to the partial as a local variable. So, if you have a partial named _albums and an instance variable named @albums, you ll get a local variable called albums inside the partial. Our form example doesn t do a very good job of illustrating this feature. Consider instead the following example, where we have all the necessary ingredients: an instance variable and a partial, both named articles.
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class ArticlesController < ApplicationController def index @articles = Article.find(:all) end end
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Here s a list template (app/views/articles/list.rhtml) that renders the _articles partial:
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<h1>Articles</h1> <%= render :partial => 'articles' %>
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And here s the _articles partial (app/views/articles/_articles.rhtml):
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<ul> <% for article in articles %> <li><%= article.title %></li> <% end %> </ul>
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The @articles instance variable that was initialized in the index action is available in the partial as the local variable articles because the partial is named articles. See the pattern here
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CHAPTER 6 ACTION PACK: WORKING WITH THE VIEW AND THE CONTROLLER
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This behavior might seem a little too magical for purists, but it s a great example of how Rails goes out of its way to save you the tedium of implementing common patterns. Still, for the purists among you, and for those rare occasions when you need to use configuration over convention, Rails makes it possible to explicitly set local template variables.
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Explicit Local Variable Assignment in Partials
Automatic assignment is not the only way to assign local variables inside a partial template. The render method also accepts a hash of local variables identified by the :locals symbol. Imagine we have two instance variables: one containing a collection of articles and another one containing a collection of comments.
<%= render :partial => 'articles', :locals => { :comments => @comments } %>
Any number of local variables can be assigned in this way, and any object can be set as the value. In the preceding example, not only will the partial have access to the @articles collection through the automatically assigned articles variable, it will also have access to the contents of the @comments collection using the local variable comments.
Rendering a Collection of Partials
Our first example of rendering the articles partial can actually be improved a bit. Notice how we re iterating over the articles list inside the partial Well, this is a common enough pattern that Rails has a better way of dealing with it. It s called rendering a collection of partials. Here s how we would rewrite the list.rhtml file from the example:
<h1>Articles</h1> <ul> <%= render :partial => 'article', :collection => @articles %> </ul>
And here s how we would rewrite the _article.rhtml partial:
<li><%= article.title %></li>
Notice that we renamed the articles partial from the plural to the singular. Recalling what you know about automatic local variable assignment, this is because we want to deal with a single article. When you render using a collection, as we ve done here, Rails will iterate over the given collection and yield the contents of the partial once for each iteration. The local variable article will become available inside the partial by virtue of the partial s name.
CHAPTER 6 ACTION PACK: WORKING WITH THE VIEW AND THE CONTROLLER
Adding the Login and Logout Actions
Our users controller needs to serve another important function: controlling the logging in and out of users. To accomplish this, we ll create two new actions: login and logout. The login action has an associated view template; the logout action does not. Listing 6-12 shows the new actions.
Listing 6-12. Login and Logout Actions Added to app/controllers/users_controller.rb
def login if request.post if user = User.authenticate(params[:login], params[:password]) session[:user_id] = user.id redirect_to events_url else flash[:notice] = 'Invalid login/password combination' end end end def logout session[:user_id] = nil redirect_to login_url end
We ll take a closer look at the login and logout actions soon, but before we go any further, we need to take a minute to talk about sessions.
Lying in State
Here s the thing: HTTP is stateless. In short, that means that each and every request you make across the HTTP protocol is autonomous. The web server has no idea that it has talked to your browser before; each request is like a blind date. Given this tidbit of information, you might be wondering how you can stay logged in to a given site. How can the application remember that you re logged in if HTTP is stateless The answer is that we fake state. You ve no doubt heard of browser cookies. In order to simulate state atop HTTP, Rails uses cookies. When the first request comes in, Rails sets a cookie on the client browser. The browser remembers the cookie locally and sends it along with each subsequent request. The result is that Rails is able to match the cookie that comes along in the request with session data stored on the server.
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