4: Working with Controllers in Java

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4: Working with Controllers
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The goal of the typical controller action is to render a view template that is, to fill out the template and hand the results, usually an HTML document, back to the server for delivery to the client Oddly at least it might strike you as a bit odd, though not illogical you don t actually need to define a controller action, as long as you ve got a template that matches the action name You can try this out in under-the-hood mode Go into app/controller/demo_ controllerrb, and delete the index action so that the file will look empty, like this:
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class DemoController < ApplicationController end
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Don t delete app/views/demo/indexhtmlhaml, and then try the console exercise (Dispatcherdispatch and all that) again You ll see the same result By the way, make sure you reload the console when you make changes it doesn t react to changes in source code automatically The easiest way to reload the console is simply to type reload! But be aware that any existing instances of Active Record objects that you re holding on to will also need to be reloaded (using their individual reload methods) Sometimes it s simpler to just exit the console and start it up again
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Rails knows that when it gets a request for the index action of the demo controller, what really matters is handing something back to the server So if there s no index action in the controller file, Rails shrugs and says, Well, let s just assume that if there were an index action, it would be empty anyway, and I d just render indexhtmlhaml So that s what I ll do You can learn something from an empty controller action, though Let s go back to this version of the demo controller:
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class DemoController < ApplicationController def index end end
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What you learn from seeing the empty action is that, at the end of every controller action, if nothing else is specified, the default behavior is to render the template whose name matches the name of the controller and action, which in this case means app/views/demo/indexhtmlhaml
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In other words, every controller action has an implicit render command in it And
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render is a real method You could write the preceding example like this:
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def index render "demo/index" end
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You don t have to, though, because it s assumed that it s what you want, and that is part of what Rails people are talking about when they discuss convention over configuration Don t force the developer to add code to accomplish something that can be assumed to be a certain way The render command, however, does more than just provide a way of telling Rails to do what it was going to do anyway
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Rendering a template is like putting on a shirt: If you don t like the first one you find in your closet the default, so to speak you can reach for another one and put it on instead If a controller action doesn t want to render its default template, it can render a different one by calling the render method explicitly Any template file in the app/views directory tree is available (Actually, that s not exactly true Any template on the whole system is available!) But why would you want your controller action to render a template other than its default There are several reasons, and by looking at some of them, we can cover all of the handy features of the controller s render method
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433 Rendering Another Action s Template
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A common reason for rendering an entirely different template is to redisplay a form, when it gets submitted with invalid data and needs correction In such circumstances, the usual web strategy is to redisplay the form with the submitted data, and trigger the simultaneous display of some error information, so that the user can correct the form and resubmit The reason that process involves rendering another template is that the action that processes the form and the action that displays the form may be and often are different from each other Therefore, the action that processes the form needs a way to redisplay the original (form) template, instead of treating the form submission as successful and moving on to whatever the next screen might be
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